pgsql: Refactor the way code is shared between some range type function

Refactor the way code is shared between some range type functions.

Functions like range_eq, range_before etc. are exposed at the SQL-level, but
they're also used internally by the GiST consistent support function. The
code sharing was done by a hack, TrickFunctionCall2, which relied on the
knowledge that all the functions used fn_extra the same way. This commit
splits the functions into internal versions that take a TypeCacheEntry as
argument, and thin wrappers to expose the functions at the SQL-level. The
internal versions can then be called directly and in a less hacky way from
the GiST consistent function.

This is just cosmetic, but backpatch to 9.2 anyway, to avoid having a
different version of this code in the 9.2 branch. That would make
backpatching fixes in this area more difficult.

Alexander Korotkov

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/79c49131fa59069c70bda107c082199e9f8bd880

Modified Files
--------------
src/backend/utils/adt/rangetypes.c | 219 ++++++++++++++++++++-----------
src/backend/utils/adt/rangetypes_gist.c | 168 ++++++++----------------
src/include/utils/rangetypes.h | 24 ++++
3 files changed, 220 insertions(+), 191 deletions(-)