Make Rubinius.add_defn_method return the name of the def'd method, as a symbol

This makes the Rubinius behavior compatible with MRI 2.1,
which now sports a specified return value for def.

>> (def foo; end) #=> :foo

See RubySpec:
  https://github.com/rubyspec/rubyspec/commit/e567d2458e46c12e50c85394f509f3b5a5688e11
Update test fixtures that relied on CompiledCode returned from def.

Some of the spec/core test fixtures were relying on the CompiledCode
instance getting returned from a method definition.

These fixtures were updated to fetch the CompiledCode instance from
the symbol, using instance_method to get the UnboundMethod instance,
which in turn holds the CompiledCode instance as its executable.

>> def foo; end #=> :foo
>> instance_method(:foo).executable #=> #<Rubinius::CompiledCode ...>
Merge pull request #2868 from jemc/def_return_symbol

Method def should return Symbol instead of CompiledCode