Add :time command

[qsctr]

Adds a :time command in the REPL for measuring the evaluation time of an expression. Uses criterion-measurement to do measurements.

The analysis that we do is very crude compared to what the full criterion package does (we basically just calculate the average times), but criterion pulls in much more dependencies so I don't think it's worth adding it.

Closes #1415.

[yav]

@qsctr could you add an entry to the CHANGES.md to record that we now have this. I forgot to mention it when reviewing the FFI PR, but we should an entry for the FFI there as well.

[yav]

I think it looks good. The only idea I had (but I am not sure it is really needed) is that it might be nice if the :time command binds the it variable with the result of the timing (e.g., using bindItVariableVal. Not sure if it should bind a record, or just a single number, or if this is needed at all, but my thinking was that if we have that (and also added some sort of "quiet" mode where the timings are just bound to it but not printed), then we could try to write benchmarking test cases checking that the times are within some range

[yav]

LGTM