pgsql: Move Timestamp/Interval typedefs and basic macros into datatype/

Move Timestamp/Interval typedefs and basic macros into datatype/timestamp.h.

As per my recent proposal, this refactors things so that these typedefs and
macros are available in a header that can be included in frontend-ish code.
I also changed various headers that were undesirably including
utils/timestamp.h to include datatype/timestamp.h instead. Unsurprisingly,
this showed that half the system was getting utils/timestamp.h by way of
xlog.h.

No actual code changes here, just header refactoring.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/a7801b62f21bd051444bd1119cd3745ecc8e14ec

Modified Files
--------------
contrib/btree_gist/btree_utils_num.c | 1 +
contrib/spi/moddatetime.c | 1 +
doc/src/sgml/xfunc.sgml | 4 +-
src/backend/access/transam/twophase.c | 1 +
src/backend/access/transam/xact.c | 1 +
src/backend/access/transam/xlog.c | 1 +
src/backend/commands/analyze.c | 1 +
src/backend/commands/async.c | 1 +
src/backend/commands/prepare.c | 1 +
src/backend/commands/user.c | 1 +
src/backend/commands/vacuumlazy.c | 1 +
src/backend/commands/variable.c | 1 +
src/backend/libpq/crypt.c | 1 +
src/backend/parser/parse_node.c | 1 +
src/backend/postmaster/autovacuum.c | 1 +
src/backend/postmaster/pgstat.c | 1 +
src/backend/replication/walreceiver.c | 1 +
src/backend/replication/walsender.c | 1 +
src/backend/storage/buffer/bufmgr.c | 1 +
src/backend/storage/ipc/standby.c | 1 +
src/backend/storage/lmgr/proc.c | 1 +
src/backend/tcop/postgres.c | 1 +
src/backend/utils/adt/date.c | 1 +
src/backend/utils/adt/genfile.c | 1 +
src/backend/utils/adt/misc.c | 5 +-
src/backend/utils/adt/nabstime.c | 1 +
src/backend/utils/adt/pgstatfuncs.c | 5 +-
src/backend/utils/adt/selfuncs.c | 1 +
src/backend/utils/mmgr/portalmem.c | 1 +
src/include/access/gist_private.h | 1 +
src/include/access/xlog.h | 2 +-
src/include/access/xlog_internal.h | 2 +
src/include/commands/sequence.h | 3 +-
src/include/datatype/timestamp.h | 163 +++++++++++++++++++++++++++++++++
src/include/libpq/libpq-be.h | 2 +-
src/include/pgstat.h | 3 +-
src/include/replication/walprotocol.h | 2 +-
src/include/replication/walsender.h | 1 +
src/include/storage/proc.h | 2 +-
src/include/utils/datetime.h | 29 +------
src/include/utils/nabstime.h | 3 +-
src/include/utils/portal.h | 2 +-
src/include/utils/timestamp.h | 108 +---------------------
43 files changed, 213 insertions(+), 150 deletions(-)