Use MRI logic for generating tempfile names in Tempfile.

This change does the following:

* Adds missing Ruby logic by defining Tempfile.create and
  including Dir::Tmpname into Tempfile. These changes happen in
  the new lib/ruby/shared/tempfile.rb.
* Invokes #create included from Tmpname to generate the filename,
  rather than using our own filename logic. This allows overriding
  both #create and #make_tempname. It also abstracts all name
  logic to Ruby's tmpdir.rb.
* Eliminate unused Reaper logic.
* Eliminate unused temp dir finding logic.
* Fixes #677
Merge remote-tracking branch 'github/master'