Support frame introspection

The approach in this changeset is to attach frame args
to every call site via stackmap args.  We make sure that
every callsite is a patchpoint, and serialize the symbol
table into its arguments.

In this patch, that is just used for supporting the locals()
method.

It's currently disabled since it has a hugely negative impact
on performance (LLVM takes much longer to JIT with all the extra
function arguments).