Rename boolean defines from `true`/`false` to `1`/`0` for consistency, prefix debug flags with `WREN_DEBUG_`, add `WREN_USE_LIB_IO` flag to guard IO library loading, and wrap IO module includes and function declarations in conditional compilation blocks.
Replace monolithic corelib.wren with per-class builtin/*.wren files and generate_builtins.py script. Move IO class definition and its native writeString_ method from wren_core.c to new wren_io.c, adding wrenGetArgumentString and wrenDefineStaticMethod to the public API. Update Makefile target from corelib to builtin and remove generate_corelib.py.