Revert "Merge pull request #2868 from jemc/def_return_symbol"

We only want to change the API exposing the behavior that MRI changed. We
don't want to change our own API.

Also need to rewrite these specs. Many spec descriptions are insufficient (eg
"has the correct value" is not an acceptable spec).

See later commits for the fixes.

This reverts commit 180466bf51b0a79ef2888e2797aaac85f9ac6714, reversing
changes made to 7c6c45276806d3b66c179193668fda491b6ddba8.
Updated CI specs to RubySpec 57e46a24.
Removed problematic spec.

We need a better mechanism for obtaining the comparison value.
Return the method name from def.
Updated inline cache specs.
Fixed some CompiledCode specs.
Moved benchmark/ to rubinius-benchmark repository.
Removed obsolete library/bin/rar.rb.
Fixed shebang in library/bin/irb.rb.
Fixed pre-installing gems to update bin wrapper shebang.