Add `Hash#map_values` to ActiveSupport to simplify a common pattern

Keys remain the same, but the values change.

Didn't get a chance to convert existing code, I'll skim through the code base to make use of this later this afternoon.
`transform_keys` and friends live in /core_ext/hash/keys.rb. To be consistent, this file could be called /core_ext/hash/values.rb.