pgsql: Optimizations for integer to decimal output.

Optimizations for integer to decimal output.

Using a lookup table of digit pairs reduces the number of divisions
needed, and calculating the length upfront saves some work; these
ideas are taken from the code previously committed for floats.

David Fetter, reviewed by Kyotaro Horiguchi, Tels, and me.

Discussion: https://postgr.es/m/20190924052620.GP31596%40fetter.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1fd687a035558238c0e3cab09fc22dc61a088869

Modified Files
--------------
src/backend/access/common/printsimple.c | 2 +-
src/backend/utils/adt/datetime.c | 98 ++++----
src/backend/utils/adt/int8.c | 1 -
src/backend/utils/adt/numutils.c | 405 ++++++++++++++++++--------------
src/include/utils/builtins.h | 8 +-
5 files changed, 287 insertions(+), 227 deletions(-)