sizebitvec of tsearch2, as well as identical code in several other
contrib modules.  This provided about a 20X speedup in building a
large tsearch2 index ... didn't try to measure its effects for other
operations.  Thanks to Stephan Vollmer for providing a test case.

more tests than strictly necessary, but did not feel like tracing call
paths in detail ...

getting rid of numerous ad-hoc versions that have popped up in various
places.  Shortens code and avoids conflict with Windows min() and max()
macros.

boxes. Change interface to user-defined GiST support methods union and
picksplit. Now instead of bytea struct it used special GistEntryVector
structure.