Refactor Enumerable#map to delegate to #map_with_index

[straight-shoota]

#map and map_with_index are identical except that the latter also yields an index counter.

But this counter is entirely optional and can be omitted from the block. It's possible to call #map_with_index with exactly the same signature as #map.

["foo", "bar"].map_with_index do |e|
  e.upcase
end # => ["FOO", "BAR"]

["foo", "bar"].map do |e|
  e.upcase
end # => ["FOO", "BAR"]

The implementation of both methods is also pretty much identical, in Enumerable as well as in any including type.
Of course, map_with_index has a counter and yields its value. But LLVM optimization happily removes it when unused.

So I think it makes sense to implement Enumerable#map by delegating to #map_with_index. This embodies the close connection between these two methods.
As a result, including types only need to override #map_with_index with a custom implementation and #map will follow suit.
Including types may still override #map with a custom implementation, of course.