Add Enumerable#transform

I want to start a discusion about this feature. The main idea of this method
is to work like `#map`, except returning a `Hash` instead of an `Array`. I think
it's an useful addition, and it simplifies the task of building hashes from
arrays - that usually requires using `#each_with_object`.

That said, I'm open to suggestions about pretty much everything; I'm not sure
this is a good naming, the examples I used for the specs are far from good
and maybe I didn't covered all the possible cases.

Also as a note, theres' an ongoing discussion on the Ruby bug tacker suggesting
to add such feature to core (https://bugs.ruby-lang.org/issues/8951).

Nitpicks and feedback are welcome.