Introduce traits

[KnightBreaker]

Every type should be able to conform to a trait, an interface/protocol like structure, not just classes that can conform to interfaces.

This is a much more powerful feature as together with retroactive extensions it would allow extending structs, enums, upcoming unions with additional shareable functionality.

Interfaces could just become a constrained subset of traits.
e.g. in Swift a protocol P: AnyObject is constrained to classes only, so it‘s like an interface, but a protocol Q {} can be used by other types as well.

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This is important to replace stuff like how ToString currently works. We may want to take a value that generically can be converted to a string but with the current system and ToString being hoisted functions it's not possible.

[janvitek]

This is on our roadmap, but a bit further down the road.