Fixes #4454
Fixes #4455

This compiles right now but shouldn't:

struct Foo
  @x : Tuple(Foo, Foo)

  def initialize(@x)
  end
end

Since a tuple is like a struct, the representation of Foo would expand infinitely. A check was missing regarding tuples and named tuples (first two commits).

Likewise, this compiles but shouldn't:

struct Bar(T)
  def initialize(@x : T)
  end
end

alias Foo = Int32 | Bar(Foo)

The problem is that the representation of Bar in memory is that of Foo, which is recursive, and since recursive aliases are represented as structs this leads to undefined behavior (random crashes).

Now, while fixing this I realized recursive aliases are not needed in the language but until (and if) we remove recursive aliases this fixes those bugs, and also fixes potential bugs regarding tuples.