Pass-by-ref for non-Copy builtins (frontend)

[Bromeon]

Enables several builtins that don't implement Copy to be passed by reference (borrowed) into engine APIs. Examples are Array, Packed*Array, Dictionary, Variant.

This lays the groundwork for future optimizations which exploit borrowing to avoid unnecessary cloning/ref-count operations, thus further reducing API call overhead.

There are some ergonomic impacts of requiring & in more places, however on the other side .clone() calls can often be avoided now. The current draft uses the Borrow trait in a few manual APIs to enable both by-value and by-ref; I'd probably use something else that uses by-value for Copy types and by-ref otherwise.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-900

[Bromeon]

Merging, as there's a sizeable amount of changes already. The "backend" changes (actual FFI passing) as well as fine-tuning the APIs like impl Borrow<T> can be done in separate PRs.