Generated emit() functions now take impl AsArg<T>

[Bromeon]

Another small signals QoL feature -- allows emit() arguments to be mostly passed like in other engine APIs, e.g.

• i32, Vector2 and other Copy types by value
• Gd, DynGd, Variant, Array, Dictionary by reference
• Strings by reference or "string" literals

AsArg for objects isn't quite as flexible as AsObjectArg, which allows derived-to-base conversions. Generalizing AsArg the obvious way creates problems in other places, such as

let obj: Gd<...> = ...;
let a = array![&obj]; // type inference fails now

So we'd need to find a solution for array! to maybe not just call push() but do something with the types first.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1150