Separate public and internal ToGodot + FromGodot traits

[Bromeon]

Introduces new internal traits EngineToGodot + EngineFromGodot. These are used by functions in generated code for engine APIs. Where available, they delegate to ToGodot + FromGodot traits via blanket impl. In addition, they have implementations for types that make sense on an engine level, but not for user-defined #[func] functions or Variant conversions.

This addresses multiple problems:

1. Type u64 currently implements ToGodot/FromGodot out of necessity:

   • It's required for engine-provided APIs, which are backed by C++ std::uint64_t and thus safe.
   • However, #[func] interacts with GDScript, which does not have an unsigned 64-bit type. The current implementation panics, which is the only instance of such behavior in otherwise infallible ToGodot::to_godot(). Silently changing value would be even worse.
   • This PR removes impl To/FromGodot for u64.

2. Similar: raw pointers like native-struct *const AudioFrame, or *const c_void used in virtual methods.

   • While not technically unsound (?), there's hardly a way to use those correctly in variant conversions or #[func]. You can just do 123.to_variant().to::<*mut Glyph>() and obtain a bad pointer.
   • Again, engine methods need them but #[func] doesn't.
   • Removing such impls also declutters the impl docs for ToGodot and FromGodot.

3. There is currently a bug: API calls cause runtime panics for some u64 values.

   • If highest bit is set, i.e. value > i64::MAX, the ToGodot validation kicks in and causes panic.
   • Itest u64_fileaccess_roundtrip tests that this works after the changes.

4. Relatedly, bitfields are represented as u64.

   • According to my brief research, there are no bitfields that currently use the highest bit, but this is possible. If that happens, then a panic could also occur.
   • A private EngineToGodot trait can handle this case without exposing error-prone conversions for public use.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1413