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