:time command for measuring evaluation time

[qsctr]

We can add a :time command to measure how long it takes to evaluate an expression. We might be able to use something like criterion-measurement for accurate timings over many iterations. This would be useful for benchmarking Cryptol code.

Note that laziness might result in the first iteration of evaluating a top-level definition being slower, so maybe the user should be careful to pre-evaluate certain things if they don't want that to be part of the measurement.