GDScript -> Rust calls: parameter and return type conversions

[Bromeon]

We need to work out rules regarding how values from and to Rust #[func] methods are passed, when invoked from GDScript. Most notably, we should decide which implicit conversions (if at all) we provide. Then, we should test those, and make sure unwanted conversions don't happen (and panic instead).

Generally I'd like an approach where we start quite restrictive, and then can loosen restrictions on a demand basis. This not only avoids bugs and makes things more explicit, but it also prevents performance traps where someone passes Array to a Vec, needing reallocation and copying each frame.

Or we keep type-safety, but provide tooling to avoid excessive user-side converting.

Examples:

GDScript	Rust	allowed
int	f32
float	i32
float	u32
String	std::string::String
StringName	std::string::String
Array	Array<i32>
Array	Array<Variant>
Array	Vec<Variant>
Array[int]	Array<i32>
Array[int]	Array<Variant>
...	Variant
...	Option<T>
...	Result<T, E>

[ttencate]

There is prior art in GDNative as well. It lets you receive usize and u8 at least, which are fallible conversions from the engine's i64 type. Especially usize is very practical to have for e.g. indices. I haven't checked, but I assume the conversion will panic if the value is out of range.

[Bromeon]

See also #263.