Reworked metadata command abbreviations.

OLD:

     metadatum - add (mda)
     metadatum - get (mdg)
     metadatum - remove (mdrm)

     metadata module - remove (mdmrm)
     metadata module - rewrite (mdmrw)
     metadata module - view (mdmv)

NEW:

     metadatum - add (mma)
     metadatum - get (mmg)
     metadatum - remove (mmrm)

     metadata module - remove (mmmrm)
     metadata module - rewrite (mmmrw)
     metadata module - view (mmmv)

This disambiguates the md* namespace: md* commands now all act on material
definition modules; mm* commands now all act on metadata.
Updated TODO.