Fix FFI generation to not depend on bindgen

[notnotmelon]

Currently the FFI generator creates .hpp files that are intended to be parsed by bindgen.
In practice the .hpp files do not compile perfectly, which is a bindgen requirement.

The C++ standard is far to complex to replicate perfectly. Additionally most users will not care about 90% of the fields on FFI structs.
We instead need to rework this repo to directly output Rust FFI psuedocode which can manually be tweaked. The psuedocode should also rely heavily on rivets::Opaque in order to ignore unwanted FFI fields while also preserving proper binary offsets.

The end result will be a completely removed dependency on bindgen.