Callables to builtin methods; Array::bsearch_by, sort_unstable_by

[Bromeon]

Adds Callable::from_variant_method(), matching Godot's Callable.create(). This can be used to invoke non-static builtin methods (e.g. on GString).

Also adds two Array APIs which now can use type-safe Rust comparators instead of their _custom variants accepting Callables:

• array.bsearch_by(|e| e.cmp(&value))
• array.sort_unstable_by(|a, b| a.cmp(b))

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1064