Xpdt

Minor changes to quiet meaningless UBSAN warnings in generated C code.

Make sure `_impl.h` header goes into same dir as C file, since it's a private part of the implementation and included as `include "foo_impl.h"`

Add the ability to write C headers into a different dir than the C file.

Lots of quality of life improvements..
- The registry (which assigns numbers to each message type) can now be automatically updated. So it doesn't fail with baffling errors when you add a new type and forget to update the registry.
- `xpdt --help` lists the available languages
- All files required for a single language are generated in a single invocation now. This prevents race conditions with registry updates, but it also just makes things more efficient and easy to use since the inputs aren't being parsed multiple times.
- Provided C support code has been improved to put buf/len parameters back in the conventional order.
- Also added xfilemap C support code, and a new helper functions
- Fixed a bug in C code generation for modules without a registry

Woops. Fixed mypy error in previous release.

Recover from malformed inputs in enum stream by yielding an exception.