Add `map import` and `map export` debug command

[dylandreimerink]

This issue proposes to add a map import {file path} and a map export {file path} command to import/export the contents of maps. At this moment, if you want to setup a map for a test you have to add the map contents one entry at a time via a macro. It would be nice if we could import the contents of a large map from a file.

Once we have the file format we can also export maps from the kernel by converting bpftool output or creating our own commands to capture the contents of maps.