Provide an initial implementation of named measurements (quil-lang/quil#90) to shake out design issues

[antalsz]

In quil-lang/quil#90, we proposed adding named measurements to Quil, allowing the user to write something like

# Pick a syntax, any syntax
MEASURE[midcircuit] 0 ro
MEASURE!midcircuit 0 ro

to get variant MEASURE calibrations. In order to ensure that that's the right design, we should add that to quil-rs and shake out the details.

The relevant changes are also submitted to the spec as a Quil-T extension in quil-lang/quil#97.