Orcfort: Druid codexes are now easier to craft (#13)

Previously codexes required raw mithril boulders, which may
not even be available depending upon embark. It now requires
mithril powder, which orcs could already create from boulders,
but also from grinding down bars.

Likewise the requirement on saplings has been removed. Yes,
your orcs *may* be tree-farmers, but there's a good chance they
won't.

A thematic upgrade in the future may have it depend upon a
variety of plants, or more likely, the souls of elves. ;)