crucible-mir: Add *IO entrypoints for each memory model operation (#…

…1107)

A follow-up to #1105/#1106. I found myself needing `subanyMirRefIO`,
`subfieldMirRefIO`, and `subjustMirRefIO` in an upcoming SAW patch, but to
avoid having to make a bunch of little patches each time I need more things
from the `crucible-mir` memory model, I opted to define `*IO` functions for
each memory model operation that performs side effects. After this patch, it
should be straightforward to call anything in the `crucible-mir` memory model
without too much trouble.