Further static safeguard against creating instances of types with `no_constructor` in Rust

@Bromeon mentioned on Discord that it can be nice to introduce a DefaultConstructible trait for NativeClasses that don't have no_constructor, and use it as a bound for Instance::new and similar functions. Although this doesn't prevent the engine from trying to call the constructor anyway, it might be a nice-to-have on the Rust side.

This is a breaking change, and should be considered for the next minor version.

[Bromeon]

Likely not going to happen until 0.10, moving to next milestone

[chitoyuu]

Might be better suited for 0.13 to be done along with #808 and #714