Add arithmetic right-shift
Marking some signing constraints
Disassemble arithmetic right-shift nicely
Use arithmetic right-shift in imul
Make <<< a synonym of <<
Merge branch 'signed-shift' into develop

Now there is an arithmetic right-shift operator, `>>>`, that takes the place
of one of the reserved operator slots. I don't remember why I resisted this
for so long, except for some perception of lack of orthogonality. If I had a
real reason, I'm sure I'll remember it as soon as I push these commits ...