Replace the now-incompatible-with-core contrib/tsearch2 module with a

Replace the now-incompatible-with-core contrib/tsearch2 module with a
compatibility package.  This supports importing dumps from past versions
using tsearch2, and provides the old names and API for most functions
that were changed.  (rewrite(ARRAY[...]) is a glaring omission, though.)

Pavel Stehule and Tom Lane