Add `map reset` command

[dylandreimerink]

When a user uses the reset command, all of the current program state is reset(loaded context, stack, registers) but the maps stay changed. This is intentional, but in some cases a user might want to also reset the contents of one or more maps. So this issue proposes to add a map reset {map index|map name} command to clear the contents of a map.

Perhaps it is useful to also include a map reset-all command, in case one has a lot of maps to reset.