"all or none" to "all or some".

This provides much greater test coverage without eating much time.
It also makes it more likely that routine regression testing will
unearth bugs.

  version 2.39 of dectest.zip adds some new test files and because
  some existing test files were getting skipped).
* Remove two docstrings which cluttered unittest's output.
* Simplify a for-loop with a list comprehension.

  by the locals() call in the context constructor.

* Remove unnecessary properties for int, exp, and sign which duplicated
  information returned by as_tuple().

* Context.create_decimal can take a zero default just like Decimal().
* Fix typo in comment.

* Rename "trap_enablers" to just "traps".
* Simplify names of "settraps" and "setflags" to just "traps" and "flags".
* Show "capitals" in the context representation
* Simplify the Context constructor to match its repr form so that only
  the set flags and traps need to be listed.
* Representation can now be run through eval().

Improve the error message when the Decimal constructor is given a float.

The test suite no longer needs a duplicate reset_flags method.

  public.
* Removed the non-signal conditions from __all__.
* Removed the XXX comment which was resolved.
* Use ^ instead of operator.xor
* Remove the threading lock which is no longer necessary.

* Map conditions to related signals.
* Make contexts unhashable.
* Eliminate used "default" attribute in exception definitions.
* Eliminate the _filterfunc in favor of a straight list.

Docs:
* Eliminate documented references to conditions that are not signals.
* Eliminate parenthetical notes such as "1/0 --> Inf" which are no
  longer true with the new defaults.

  for lists of mixed types.
* Test that sort works.

* Change the default precision to 28 (to match VB's decimal type).

* Added test for pickling contexts
* Renamed ExceptionList to Signals (to match wording in the spec)
* Simplified Context constructor by allowing flags=None to automatically
  generate a zeroed-out flags dictionary.
* inlined _convertString() which was used only once
* _rounding_decision is private, so excluded its contants from __all__.
* added an XXX comment with concerns about subclassing signals results in
  a deviation from the spec (maybe important, maybe not).
* Taught the test_suite to determine its own directory (modeled after code
  in regrtest.py).  Enables it to be run when the current directory is not
  the test directory.
* Added a clear_flags() method to the Context API to make it easier to do
  a common operation with flags.
* Fixed the trap_enablers defaults in BasicDefaultContext to match the spec.