pgsql: Fix overflow hazards in interval input and output conversions.

Fix overflow hazards in interval input and output conversions.

DecodeInterval (interval input) was careless about integer-overflow
hazards, allowing bogus results to be obtained for sufficiently
large input values. Also, since it initially converted the input
to a "struct tm", it was impossible to produce the full range of
representable interval values.

Meanwhile, EncodeInterval (interval output) and a few other
functions could suffer failures if asked to process sufficiently
large interval values, because they also relied on being able to
represent an interval in "struct tm" which is not designed to
handle that.

Fix all this stuff by introducing new struct types that are more
fit for purpose.

While this is clearly a bug fix, it's also an API break for any
code that's calling these functions directly. So back-patching
doesn't seem wise, especially in view of the lack of field
complaints.

Joe Koshakow, editorialized a bit by me

Discussion: https://postgr.es/m/CAAvxfHff0JLYHwyBrtMx_=6wr=k2Xp+D+-X3vEhHjJYMj+mQcg@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e39f9904671082c5ad3a2c5acbdbd028fa93bf35

Modified Files
--------------
src/backend/utils/adt/datetime.c | 734 +++++++++++++++++++++------------
src/backend/utils/adt/formatting.c | 86 +++-
src/backend/utils/adt/timestamp.c | 177 ++++----
src/include/datatype/timestamp.h | 39 ++
src/include/pgtime.h | 3 +
src/include/utils/datetime.h | 6 +-
src/include/utils/timestamp.h | 5 +-
src/test/regress/expected/interval.out | 611 +++++++++++++++++++++++++++
src/test/regress/sql/interval.sql | 184 +++++++++
9 files changed, 1473 insertions(+), 372 deletions(-)