Method def should return Symbol instead of CompiledCode

For compatibility with MRI 2.1, method defs should return a symbol corresponding to the name of the def'd method.  See the commit: https://github.com/jemc/rubinius/commit/c434f257fc3ecf82737a5ae459b28f710f9de66b.

See recent RubySpec: https://github.com/rubyspec/rubyspec/commit/e567d2458e46c12e50c85394f509f3b5a5688e11

A few test fixtures in spec/core had to be updated.  See the commit: https://github.com/jemc/rubinius/commit/5db8c8ec7c5b7bf9962a204438d5b1e8babb6926.

Also, I saw that Rubinius' copy of the RubySpec hasn't been updated recently enough to include that spec.  I figured that was outside the scope of this pull request, and should be done by whatever method it is usually done.  However, I could add it to this pull request if it is desirable to do so.