Make {Enumerable,Iterator}#flat_map work with mixed element types

[HertzDevil]

Currently, the following code fails:

[[1, 2, 3], ['a', 'b']].flat_map &.itself
# Error: expected block to return Array(U) | Iterator(U) | U, not (Array(Char) | Array(Int32))

There is no reason that the block must return Arrays and Iterators of the same element type; Array#concat and #push, which this method relies on, already work with element types that differ from self's.

The Iterator form will yield the correct elements, but it uses the wrong generic type argument, breaking certain methods like #to_a:

[[1, 2, 3], ['a', 'b']].each.flat_map(&.itself).each { |x| print x } # => 123ab

[[1, 2, 3], ['a', 'b']].each.flat_map(&.itself).to_a
# Error: no overload matches 'Array(Char)#<<' with type (Char | Int32)
#
# Overloads are:
#  - Array(T)#<<(value : T)
# Couldn't find overloads for these types:
#  - Array(Char)#<<(value : Int32)

The type restriction Array(U) | Iterator(U) | U forall U is used in the block's return type. This restriction is unneeded, since whether a value should be flattened or not depends on its dynamic type, not static type. This PR fixes both methods by replacing the restrictions with typeof expressions that deduce the common element type. Fixes #5803:

array = [1, 2, [[3]]]
result = array.flat_map do |item|
  if item.is_a?(Array)
    item
  else
    [item] of typeof(array) | typeof(array.first)
  end
end
result         # => [1, 2, [3]]
typeof(result) # => Array(Array(Array(Array(Int32)) | Int32) | Array(Array(Int32)) | Array(Int32) | Int32)

[Sija]

[...] Fixes #5803:

@HertzDevil actually it doesn't, it just changes the code so it doesn't reproduce anymore, but the underlying issue is still there, that's different.

[bcardiff]

FTR The snippet of #5803 works with this patch. I agree there are cases where the module validation could still appear, but the changes proposed here make sense either way.