FileChunks. They're not really supposed to go there, and if they do, it
will cause an error on compilation. This just fixes that to be a parse
error instead of a compile error.
better path going forward to handling upgrades. Old syntax files will
stick around for compatibility purposes. Old syntax can be parsed into
valid syntax trees via the old syntax (.peg) files, and then old syntax
trees should be valid and can be upgraded via the normal code path. This
change has lots of improvements to Nomsu codegen too.
