Add deprecation framework, and do some deprecations

[njsmith]

trio.socket's sendall method is deprecated in favor of the new
high-level interface (see #291)

trio.run_in_worker_thread is renamed to
trio.run_sync_in_worker_thread (see #68), with the old name retained
for now as a deprecated alias.

[codecov]

Codecov Report

Merging #296 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #296      +/-   ##
==========================================
- Coverage   99.18%   99.18%   -0.01%     
==========================================
  Files          79       81       +2     
  Lines        9665    10144     +479     
  Branches      686      731      +45     
==========================================
+ Hits         9586    10061     +475     
- Misses         62       66       +4     
  Partials       17       17

Impacted Files	Coverage Δ
trio/_core/_traps.py	100% <ø> (ø)	⬆️
trio/tests/test_ssl.py	100% <ø> (ø)	⬆️
trio/_sync.py	100% <ø> (ø)	⬆️
trio/_deprecate.py	100% <100%> (ø)
trio/__init__.py	100% <100%> (ø)	⬆️
trio/_threads.py	100% <100%> (ø)	⬆️
trio/tests/test_socket.py	100% <100%> (ø)	⬆️
trio/tests/test_threads.py	100% <100%> (ø)	⬆️
trio/_file_io.py	100% <100%> (ø)	⬆️
trio/_socket.py	100% <100%> (ø)	⬆️
... and 11 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 38b49fa...f71aba7. Read the comment docs.

[njsmith]

The codecov results here don't make any sense at all. The coverage clearly isn't even aligned with the underlying source code ­– there are blank lines marked as "covered", and lines with code marked as "no code here".

And I ran the coverage locally, and I don't see anything unexpected in test_util.py or _highlevel_socket.py. I suspect that some of our CI builders are sending corrupted coverage data, or something like that.