Union type restrictions with multiple free vars

[HertzDevil]

#10267 disallows unions in type restrictions that refer to more than one free var. However, some unions with multiple free vars are in fact solvable:

def foo(x : T, y : U, z : T | U) forall T, U
end

foo(1, 'a', 1)   # okay, T = Int32, U = Char
foo(1, 'a', 'a') # okay, T = Int32, U = Char
foo(1, 1, 1)     # okay, T = Int32, U = Int32

That PR also doesn't raise on free vars that aren't directly under the union type, so the following oddity happens on 0.36.1: (contrast this to #8973 and #10370)

def foo(x : Array(T) | T) forall T
  T
end

def bar(x : T | Array(T)) forall T
  T
end

foo([1]) # => Int32
bar([1]) # => Array(Int32)

foo([[1]] || [[1], 1]) # => (Array(Int32) | Int32)
# note that typeof(x) <= (Array(T) | T) is false in this case; somewhere
# Crystal considers Array(X | Y) to be distributive over the union, turning
# Array(Array(Int32) | Int32) into Array(Array(Int32)) | Array(Int32)

bar([[1]] || [[1], 1]) # => (Array(Int32) | Int32)

Until we figure out how to unify those kinds of free vars, I think we should disallow any union type restrictions that use more than one free var, whether they are identical or not, and whether they appear as generic type / metaclass arguments or not.