Enable writing module code directly to file

[lizard-boy]

Summary

Enables generating code directly into files:

with open('code.py', 'w') as f:
    module.write_code(f)

rather than

code = module.code
with open('code.py', 'w') as f:
    f.write(code)

It does this with a subclass of CodegenState that avoids building in memory:

• the tokens: List[str] field of CodegenState
• the "".join(state.tokens) string of code

Test Plan

• hatch run test passes
• we could add some tests that compare the contents of written files written directly vs indirectly

Notes

• CodegenState.pop_trailing_newline() isn't supported because the token is already written to the file. this is fixable if desired.