Need to make setattr() stealing

xdecref is a fair bit slower than decref (especially as it's implemented right
now as a helper function), but I think this will be ok once we get the llvm
jit up and running with refcounting.

First issue is that the assembly logging previously worked
on the assembler buffer, which is reasonable but has issues with the bjit
which will update the assembly post-relocation (ie by initializing the patchpoints).

Also add some extra flags to separate commenting for rewriter-level comments
("this is the beginning of a setattr") and the new bjit ones ("this is
a set local").

not sure about the code they emit though, since right now it's too large.