debugger: add a friendlier user interface.

Just a collection of Lua functions that make it a little easier to run
commands interactively.
rtl/debug: clean debug state machine.

Assign the outputs as combinational logic otherwise the additional
latency of the next cycle + memory access latency makes it hard to plan
ahead.
verif/debug: set SO_REUSEADDR before bind()

Otherwise it doesn't have any effect and we end up with sockets stuck in
TIME_WAIT.