s|=s, s&=s, s-=s, or s^=s).  Add related tests.

* Improve names for several variables and functions.

* Provide alternate table access functions (next, contains, add, and discard)
  that work with an entry argument instead of just a key.  This improves
  set-vs-set operations because we already have a hash value for each key
  and can avoid unnecessary calls to PyObject_Hash().  Provides a 5% to 20%
  speed-up for quick hashing elements like strings and integers.  Provides
  much more substantial improvements for slow hashing elements like tuples
  or objects defining a custom __hash__() function.

* Have difference operations resize() when 1/5 of the elements are dummies.
  Formerly, it was 1/6.  The new ratio triggers less frequently and only
  in cases that it can resize quicker and with greater benefit.  The right
  answer is probably either 1/4, 1/5, or 1/6.  Picked the middle value for
  an even trade-off between resize time and the space/time costs of dummy
  entries.