Add SingleOps.__round__ and optimizations in Builtin.round.

[mickp]

Testing

>>> import System
>>> v = 987.654321
>>> assert round(System.Double(v)) == round(System.Single(v))
>>> print(round(System.Double(v), 3), round(System.Single(v), 3))
987.654 987.654

[mickp]

I attempted to add assertions in tests/suite/test_builtinfunc.py, but I can't get those tests to run: they fail with

RUNNING IRONPYTHON TESTS USING THESE FLAGS:
Traceback (most recent call last):
  File "run.py", line 330, in <module>
  File "run.py", line 295, in runTest
  File "run.py", line 241, in runTestSlow
  File "run.py", line 226, in run_one_command
AttributeError: 'module' object has no attribute 'popen3'

[slozier]

Thanks for the PR. I think it generally seems fine. I won't have time to properly review/test this for a couple weeks, but I'll get around to it as soon as I can.

Not sure why the test changes aren't working. I've been doing some structural changes recently so I may be to blame...

So I don't forget... I want to check are how it compares to "prior art" such as np.float32.