Be more strict about unicode/bytes conversions

[jcrist]

Previously this library relied upon duck-typed behavior, instead of following precise schemas for the conversions between ctypes structs and dictionaries of python objects. This led to unicode decode errors, as bytestrings not representing human readable content were being decoded.

This fixes this issue in the following ways:

• Add to_dict methods to each relevant struct class that knows exactly which fields are bytes and which are strings, and handles all conversions appropriately.
• Make ensure_bytes and ensure_string more strict in terms of input and output.

[jcrist]

cc @brendancol

[martindurant]

The py3 test run aborted - does it succeed for you?

[jcrist]

It does on the recent release of distributed, but not on distributed master. Looks like https://github.com/dask/distributed/pull/1594 broke something. cc @pitrou, @mrocklin.

[pitrou]

If you want to diagnose this a bit further, I suggest:

• add the -s option to py.test to display all stdout/stderr output immediately
• use the pytest-timeout plugin to print a traceback for timed out tests

[jcrist]

Ok, after removing the failing dask tests, this now passes.

[martindurant]

+1 for the green