Require right-hand side of one-to-many assignments to be Indexable

[HertzDevil]

Fixes #11414. The entire right-hand side needs to be an Indexable, not a union of Indexable and non-Indexable types (maybe this could be relaxed like in #11145, but since the literal expander cannot decide whether the RHS can possibly be such a union, every MultiAssign will end up having a runtime is_a?(::Indexable) check).

class Foo
  def [](index)
  end

  def size
    3
  end
end

a, b, c = Foo.new  # => [nil, nil, nil]
d, *e, f = Foo.new # Error: right-hand side of one-to-many assignment must be an Indexable, not Foo

If -Dstrict_multi_assign is provided, the first assignment also produces the same compilation error.

The first commit makes Enumerable and Indexable built-in types. This does little other than making sure these modules exist even with an empty prelude; nothing else is done in Crystal::Program (in particular Indexable does not even include Enumerable there), and the standard library continues to be responsible for those additional definitions.