Support type splats inside explicit Unions in type restrictions

[HertzDevil]

Implements #8520 (comment). The main use case is when the type being splatted is a generic type var of the enclosing type:

class Foo(*T)
  def self.foo(x : Union(*T))
  end
end

Foo(Int32, String).foo(1) # ok, `Union(*T)` resolves to `Union(Int32, String)`

But it can be any Tuple too:

def foo(x : Union(*{Int32, String})); end
# same as:
def foo(x : Union(Int32, String)); end

This behaviour is consistent with other generic types. Also, unlike other generics multiple splats are allowed inside Union, because they must not be free vars (Union(*T) forall T will never match anything).

[asterite]

Wow, you are on fire!