Fixed getting classes from Tracepoint objects

Apparently when using RBASIC_CLASS() sometimes Ruby returns something
that _looks_ like a VALUE but isn't. This results in Ruby sometimes
treating it as a Fixnum of sorts while in other cases it just segfaults.

To work around this we're now using rb_tracearg_defined_class() to get
an object class, which seems to be the correct way of doing it (based on
digging through the MRI source code).

One small nuisance is that we now also get singleton classes which we
generally don't care about (since there's always only one of them). This
means we'll need to filter these out whenever we generate a Ruby Hash
containing the counts per class. Of course MRI doesn't provide a sane
API for this (that doesn't involve calling Ruby methods) so we instead
use the same logic as MRI's implementation of Class#singleton_class?
uses.