X86 register renaming

In the spirit of abstracting things out so we can support something other than x86, this patch series moves the debugging stub's knowledge of x86 registers into the Registers class itself.

I'm not entirely sure this works; it compiles locally, but running rr on my machine has been somewhat finnicky, and therefore I haven't yet run tests.  I'm mostly asking for feedback at this point, since this is my first rr patch.