Explicitly check C-API Handle 'checksum'.
Revert "Revert "Setup finalizer thread as an auxiliary thread""

This reverts commit 75537c1417abb5dfcefca25441a1271d7a20216e.
Only make Rubinius FFI available via require.
Removed FFI stub gems.
Temporarily disable running Ruby finalizers.
Don't swallow potential bugs with Data marking and freeing.
Added pre-installed FFI 1.2.0 gem.

Bundling this gem because FFI gem maintainer hasn't answered my
query about releasing a version of 1.2 that installs on Rubinius.
Don't deadlock adding objects to finalizer queue.
Load Rubinius FFI via require.
Disable FFI CI specs.

Two issues:

* We do not load RubyGems to run the specs.
* We are using the FFI gem, so the gems specs should be run to
  ensure that the gem functions on Rubinius.
Temporarily tag failing OS finalizer specs.