overhaul runtime stack manipulation

[billhails]

depends on #83

• Probably first do some profiling to verify that performance is hampered by all of the stack copying that goes on when reinstating continuations etc. I can't see how it isn't though.
• Investigate options for only, or mostly only, manipulating pointers to stacks instead. call/cc will complicate this of course, because continuations are re-usable.