Unfold type alias before splatting on block parameters

[soutaro]

Steep supports block parameter auto-splat on tuples and arrays.

# @type var array: Array[[Integer, String]]
array = ...

array.each do |x, y|
  x + 1      # x is Integer
  y + ""     # y is String
end

This PR is to let the array element a type alias -- like Array[pair] where type pair = [Integer, String].

Note that this doesn't support #to_ary conversion, or the case the argument is a subclass of Array.