Basic support for Kernel.trace_var/untrace_var.

There are three catches to this:

1. Rubinius::Globals.set_hook is used under the hood but only provides support
   for a single tracer per global variable. MRI on the other hand allows for
   many.

2. Kernel.trace_var can take a String which is then evaluated in the context of
   the caller. I'm not sure how to achieve this in Rbx so for the time being it
   uses just `eval('...')`

3. Kernel.untrace_var allows you to remove specific tracers based on the 2nd
   argument. Since Rubinius only allows for single tracers to be defined the
   2nd argument is ignored.
Updated specs for Kernel.trace_var/untrace_var.