Fix check_if_positive_fixnums

This check passed wrongly when neither of the arguments was a fixnum.
This changes the check to make sure that or'ing and and'ing the tags
both result in a tag equal to the fixnum tag.

Fixes #2153
Added raytracer benchmark
Remove stale comment
Add a spec for Proc#== with rb_proc_new
Also consider @bound_method for equality
Add specs for Proc#{inspect,source_location} with rb_proc_new
Remove extra spaces around @ for consistency
Correctly override #source_location at Proc::Method

Also, to make it happen, this versionizes Proc::Method#inspect.
Fix Proc#source_location because @block can be nil