Introduce REPL mode

[djolertrk]

Introduce pure gdb-like REPL mode.

(miden-debug) b file:line
(miden-debug) continue
...

[djolertrk]

cc @bitwalker @greenhat

This is low priority.

[djolertrk]

Already implemented as part of #17, so will be merged along with the variable's tracking feature.