Introduce continuations back into scm-slang

[s-kybound]

The changes in #165 have turned scm-slang into a conductor-compatible language evaluator, which is good news. However, the migration targets Scheme chapter 1, which loses prior work to enable continuations and macros in the Scheme implementation.

Implementing continuations will require a new builtin callcc as well as a continuation datastructure that contains a saved Control, Stash and Environment stack, that will be restored once the continuation is applied. The details can be found in this paper: https://icfp24.sigplan.org/details/scheme-2024-papers/2/Beyond-SICP-Design-and-Implementation-of-a-Notional-Machine-for-Scheme