Traits for Static Dispatch

[npc1054657282]

@Verafahn

Allow |T: trait| syntax. When the inferred T does not satisfy trait, it leads to compilation errors. I think this approach is more intuitive than simply writing trait in the type position.

The new grammar is unnecessary. Just:

fn (arg: |T|) R: {
    if (!satisfySomeTrait(T)) comptime unreachable;
    break :R T;
} {
    ...
}