C Interoperability

# C Interoperability

Tomo is intended to be used as a complete, standalone programming language, but
it's also meant to be easy to integrate with existing C libraries. In order to
make this possible, there are a few tools available.

## Using C Libraries 

In order to link against a compiled shared library, you can use `use -lfoo` to
cause Tomo to add `-lfoo` to the linker flags when compiling your final
executable. You can also use `use <foo.h>` or `use ./foo.h` to cause Tomo to
insert a corresponding `#include` when compiling your code.

You can also `use ./foo.c` or `use ./foo.S` to use C or assembly source files
from inside a Tomo source file.

## Inline C Code

As a final escape hatch, you can use `C_code` to add code that will be put,
verbatim in the transpiled C code generated by Tomo. There are two forms: one
that creates an expression value and one that creates a block that is executed
without evaluating to anything:

```tomo
# Inline C block:
C_code `
    printf("This is just a block that is executed without a return value\n");
`

# Inline C expression (you must specify a type)
val := C_code : Int32 `int x = 1; x + 1`
```

Inline C expressions must specify a type and they can be [compound statement
expressions](https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html). In
other words, if an inline C expression has a type, it will be enclosed
with `({ ...; })` so that you can put semicolon-terminated statements before
the final expression in their own scope if you want.