Rename things for consistency with the new "checkpoint" terminology

[njsmith]

Closes gh-157

[codecov]

Codecov Report

Merging #311 into master will increase coverage by <.01%.
The diff coverage is 99.67%.

@@            Coverage Diff             @@
##           master     #311      +/-   ##
==========================================
+ Coverage   99.24%   99.24%   +<.01%     
==========================================
  Files          84       84              
  Lines       10328    10340      +12     
  Branches      725      725              
==========================================
+ Hits        10250    10262      +12     
  Misses         61       61              
  Partials       17       17

Impacted Files	Coverage Δ
trio/_core/_io_kqueue.py	79.51% <100%> (ø)	⬆️
trio/_util.py	92.55% <100%> (ø)	⬆️
trio/_core/tests/test_ki.py	100% <100%> (ø)	⬆️
trio/_core/_traps.py	100% <100%> (ø)	⬆️
trio/tests/test_highlevel_socket.py	100% <100%> (ø)	⬆️
trio/tests/test_threads.py	100% <100%> (ø)	⬆️
trio/_core/tests/test_io.py	100% <100%> (ø)	⬆️
trio/_timeouts.py	100% <100%> (ø)	⬆️
trio/testing/__init__.py	100% <100%> (ø)	⬆️
trio/testing/_checkpoints.py	100% <100%> (ø)	⬆️
... and 26 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 3613ca6...68a14ef. Read the comment docs.