This is a follow-up of sorts to id:"8761ycc19t.fsf at qmul.ac.uk", where
Mark suggested that the part handling commands could all use the
correponding mm-* functions. I ran with the idea and wound up with
this series, which, in addition to standardizing on the mm-* functions
for everything and simplifying the implementation overall, decouples
the part commands from part buttons, which removes an entire layer
from the implementation and adds the ability to invoke part commands
with point anywhere in a part (something I often find myself wanting).
