Make debug mode usable

[ncatlin]

At the moment it just continuously sends back every tag that executes from the start of the program, which is probably excruciatingly slow enough to be useless for most applications.

Better would be to have the '*_dbg' functions call an intermediary which track time the time differences between calls.
Big difference -> probably stepping -> use the current debug_mode logic
Small difference (multiple times) -> use optimised process_block logic