String argument conversion + AsArg trait

[Bromeon]

This is a colossal PR, simply because the changes go very deep into code generation and touch all the foundations:

• Add AsArg<T> trait for generic passing of string arguments.
• Change codegen to use impl AsArg<T> for strings.
• Refactor the very messy parameter/lifetime/extender signature generation.
• Generalize AsArg<T> for generic argument passing inside Array<T> and Packed*Array.
• Update hundreds of occurrences in itest, examples as well as higher-level internal code.

TLDR: instead of

godot_method("thing".into());
godot_method(gstring.clone());

you now write:

godot_method("thing");
godot_method(&gstring);

There are still some rough edges, but it's a mergeable start, and I'd like to iteratively improve it based on user feedback. Since the whole argument passing has been fundamentally reworked lately (object arguments in #800 + reference passing in #900 and #906), there is a lot that can be made more ergonomic and consistent in the future.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-940