Interactive debugger

[Idorobots]

Given that the trampoline is made explicit with #190, it should now be fairly easy to introduce a version that allows stepping through the continuations in the REPL.

A call to (breakpoint) will yield a marked resumable and cause the original trampoline to exit. That return is trapped by the debugger trampoline, which allows a step-by-step execution with REPL commands:

• ;step - executes the resumable once.
• ;step-over - executes the resumable until a new function call appears on the call trace*
• ;trace N - show the N most recent calls in the call trace
• ;run'- resumes with the usual trampoline
• ;back - resets the computation to the last breakpoint
• ;reset - resets the computation to beginning

For inspecting the stack-trace, a separate call-trace can be used - should store the active function frames in a ring buffer in debug-compiled code (#197).

REPL should also trap Racket execution breaks and treat these as breakpoints.