swift234

[utils] Python 3 support + tidies in check-incremental

* Use print as a function, not as a statement
* Commonise the code for compiling and getting binary statistics into
  a reusable function
* Pull out `VERBOSE` and `NUM_ITERATIONS` as top-level constants, to
  make it clear where/how to change them.
* Use a docstring rather than a block comment.
* In the original version, `"Iteration {}:".format(iteration)` would
  index from 0; in human-readable output, index 1 is more sensible.