Add library documentation

libraries.md

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+# Tomo Library/Module Design
+
+There are two ways to "import" code that is defined elsewhere: local files from
+the same project and shared library objects from another project. The first
+type of import (local files) is necessary for splitting large projects into
+smaller components for ease of understanding and compilation speed. The second
+type of import (shared libraries) is to allow you to install third party
+libraries or frameworks that can be used across many projects.
+
+## Local Imports
+
+To see how local imports work, let's look at a simple file:
+
+```
+// File: foo.tm
+my_variable := 123
+```
+
+When this file is compiled to a static object file by `tomo -c foo.tm`, it
+produces the following C header file and C source file:
+
+```c
+// File: foo.tm.h
+#pragma once
+#include <tomo/tomo.h>
+
+extern Int_t foo$my_variable;
+```
+
+```c
+// File: foo.tm.c
+#include <tomo/tomo.h>
+#include "foo.tm.h"
+
+Int_t foo$my_variable = I64(123);
+```
+
+Notice that the symbols defined here (`foo$my_variable`) use a file-based
+prefix that includes a dollar sign. C compilers support an extension that
+allows dollar signs in identifiers, and this allows us to use guaranteed-unique
+prefixes so symbols from one file don't have naming collisions with symbols
+in another file.
+
+The C file is compiled by invoking the C compiler with something like: `cc
+<flags...> -c foo.tm.c -o foo.tm.o`
+
+Now, what happens if we want to _use_ the compiled object file?
+
+```
+// File: baz.tm
+foo := use ./foo
+
+func say_stuff():
+    say("I got {foo.my_variable} from foo")
+
+func main():
+    say_stuff()
+```
+
+If I want to run `baz.tm` with `tomo baz.tm` then this transpiles to:
+
+```c
+// File: baz.tm.h
+#pragma once
+#include <tomo/tomo.h>
+#include "./foo.tm.h"
+
+void baz$say_stuff();
+void baz$main();
+```
+
+```c
+// File: baz.tm.c
+#include <tomo/tomo.h>
+#include "baz.tm.h"
+
+public void baz$say_stuff()
+{
+  say(CORD_all("I got ", Int$as_text(&foo$my_variable, no, &$Int), " from foo"));
+}
+
+public void baz$main()
+{
+    baz$say_stuff();
+}
+```
+
+Then `baz.tm.o` is compiled to a static object with `cc <flags...> -c baz.tm.c
+-o baz.tm.o`.
+
+Next, we need to create an actual executable file that will invoke `baz$main()`
+(with any command line arguments). To do that, we create a small wrapper
+program:
+
+```c
+// File: /tmp/program.c
+#include <tomo/tomo.h>
+#include "baz.tm.h"
+ 
+int main(int argc, char *argv[])
+{
+    tomo_init();
+    if (argc > 1)
+        errx(1, "This program doesn't take any arguments.");
+    baz$main();
+    return 0;
+}
+```
+
+This program is compiled with the already-built object files to produce an
+executable binary called `foo` like this: `cc <flags...> /tmp/program.c
+foo.tm.o baz.tm.o -o baz`
+
+Finally, the resulting binary can be executed to actually run the program!
+
+
+## Shared Library Imports
+
+Now, what's the story with shared library imports? The equivalent process for C
+is to create a `.so` or `.dll` file. In order to build a shared library, you
+run the command `tomo -s=qux.1.2.3 file1.tm file2.tm...`. Each specified file
+will have its `.o` static object file compiled, along with its dependencies,
+and all of the resulting `.o` files will be linked together by `tomo` with a
+command like `cc <flags...> -Wl,-soname=libqux.1.2.3.so -shared file1.tm.o
+file2.tm.o dep1.tm.o ... -o libqux.1.2.3.so`. The specified files must not
+define the same public symbols as each other, since `foo` will now be treated
+as a single namespace that holds all the symbols from each of the given files.
+
+### Symbol Uniqueness
+
+In the future, each of these symbols will be given an extra prefix to prevent
+namespace collisions and a standalone header file will be built that defines
+every public symbol in the library in a way that a C compiler can understand.
+In our example, running `tomo -s=qux.1.2.3 foo.tm baz.tm` would produce a
+header file like this:
+
+```c
+#pragma once
+#include <tomo.h>
+
+extern Int_t qux$1$2$3$foo$my_variable;
+extern void qux$1$2$3$baz$say_stuff();
+```
+
+### Installing
+
+Now, the components necessary to install this shared library on your computer
+are these:
+
+- The `.so` file, installed in a standard location. In our case, we will default
+  to `~/.local/share/tomo/lib/libqux.so`
+- The standalone `.h` file, installed in a standard location. We default to
+  `~/.local/share/tomo/include/qux.h`
+- All of the source `.tm` files (which store type information necessary for tomo
+  to understand what's being imported. These will be installed to
+  `~/.local/share/tomo/src/qux/`
+
+### Using Shared Libraries
+
+To use a shared library, write a statement like `use qux` with an unqualified
+name (i.e. not an absolute or relative path like `/qux` or `./qux`). When a
+program uses a shared library, that shared library gets dynamically linked to
+the executable when compiling, and all of the necessary symbol information is
+read from the source files during compilation.
+
+### Library Versioning
+
+In order to accommodate multiple versions of the same shared libraries on a
+system, users may specify a library version when compiling, for example: `tomo
+-s qux.1.2.3 foo.tm baz.tm` During installation, symlinks are created to map
+less specific version numbers to more specific version numbers. For example,
+when installing `qux.1.2.3`, links are created:
+
+- `~/.local/share/lib/tomo/libqux.1.2.so` -> `~/.local/share/lib/tomo/libqux.1.2.3.so`
+- `~/.local/share/lib/tomo/libqux.1.so` -> `~/.local/share/lib/tomo/libqux.1.2.3.so`
+- `~/.local/share/lib/tomo/libqux.so` -> `~/.local/share/lib/tomo/libqux.1.2.3.so`
+- And so on for `include/tomo/libqux.1.2.3.h` and `src/tomo/qux.1.2.3/`
+
+If there are multiple versions (e.g. `1.2.3` and `1.3.0`), then links point at
+the highest-numbered version with the necessary prefix. In this case, `qux ->
+qux.1.3.0`, `qux.1 -> qux.1.3.0`, `qux.1.3 -> qux.1.3.0`, and `qux.1.2 ->
+qux.1.2.3`.
+