Add a debugger

[hellerve]

This PR fixes #24 by adding a debugger. It’s unfortunately pretty heavy, but comes with a cool enough set of features to warrant the ~300 lines or so added.

• adjustable prompt
• can render backtraces
• can move up and down the callstack and into specific frames
• can inspect local variables and source code (with some SBCL limitations)
• can step (into, next, out) and set and activate breakpoints

cheers