support ruby Procs in jsland

Make this work:

ctx[:alert] = lambda { |m| puts m }
ctx.evaluate("alert('hai!')")

The sticky wicket here is that the ContextPrivate slot can't be used to store a pointer to the Ruby lambda: a JSFunction backed by a native method already uses that slot to stash the function pointer.

Probably still need to do some sort of JSland function name munging to include the oid of the rubyland Proc.