Implement compact_inventory

The previous implmentation was untested, broken and slow ... Now it's
reasonably fast and actually works !
Make the digpit and digtunnel automatically empty their inventories
Merge branch 'master' of https://github.com/comkieffer/terrapin

Conflicts:
	terrapin/programs/turtle/digTunnel.lua
Add a --leaves option to the treefarm program to collect leaves too.
Make libdig compact its inventory to free up space when it fills up.