Fix various proc bugs

Well, it turns out that I can't still just do merge Proc::Method into Proc....

This pull request fixes various small bugs I found while doing the merge work.

There is one caution. It is that this pull request introduces minor incompatibility with MRI.

Namely, with ruby 1.8 mode, rubinius returns valid location from Proc#source_location. On the other hand, MRI doesn't and won't be fixed...

Also, with ruby 1.9 mode, rubinius includes source location in the all of cases of Proc#inspect. On the other hand, MRI doesn't.