member descriptor ics

trying to fill in rewriting holes... for this one I had to some support for doubles and some `mov` variants in the assembler and rewriter. Kind of hacky/lazy...
I think it'd be nice to split this into the separate instructions, perhaps that can call a common base function with different parameters.  ie `movzbl()` or what not could call a generic `mov()` function.

I think it'd be good if there is as little difference between our Assembler API and what someone would be familiar with writing textually; ie if there's already a well-established interface for specifying assembly instructions, we might as well use it instead of inventing another one even if it's reasonable.

I like the assembler that V8 has: https://code.google.com/p/v8/source/browse/trunk/src/x64/assembler-x64.h, which maybe we should just take instead of reimplementing.