To/FromGodot has now specific impls for native structs; trait docs

[Bromeon]

Changes:

1. ToGodot + FromGodot had blanket impls for all *mut T and *const T pointers, converting as ints.
   • Potential footgun even if technically safe; it's very easy to get invalid pointers in from_godot() or from_variant(), and this cannot be checked.
   • Only needed because of native structures in godot::engine.
   • Solution: implement ToGodot + FromGodot directly for pointers on those types.
2. Hide internal FFI- and conversion-related traits and methods from public API.
3. More documentation around public-facing conversion traits.
4. Rename left-over godot_compatible module to godot_convert.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-457