pwkit/kwargv.py: robustify to unicode_literals
pwkit/cli/imtool.py: add 'setrect' subcommand
pwkit/msmt.py: begin work on importing this framework
pwkit/msmt.py: fix an ancient but likely-never-to-surface mistake
pwkit/msmt.py: import modernized Lval and Textual code

This is a meld of a lot of work going into implementing Lvals and Textual with
a unified math system. It took me many tries to get the Lval behavior right
and to figure out how to provide a unified math library without a lot of
redundant implementations lying around. I believe that this version gets
things pretty much as right as I can manage.
pwkit/msmt.py: tidy up comments
pwkit/msmt.py: implement algebra on Textuals
pwkit/msmt.py: parametrize the default Uval repval method
pwkit/msmt.py: docstrings