pgsql: Add overflow-safe math inline functions for unsigned integers

Add overflow-safe math inline functions for unsigned integers

Similarly to the signed versions added in 4d6ad31, this adds a set of
inline functions for overflow checks with unsigned integers, including
uint16, uint32 and uint64. This relies on compiler built-in overflow
checks by default if available. The behavior of unsigned integers is
well-defined so the fallback implementations checks are simple for
additions and subtractions. Multiplications avoid division-based checks
which are expensive if possible, still this can happen for uint64 if
128-bit integers are not available.

While on it, the code in common/int.h is reorganized to avoid too many
duplicated comments. The new macros will be used in a follow-up patch.

All thanks to Andres Freund for the input provided.

Author: Fabien Coelho, Michael Paquier
Discussion: https://postgr.es/m/20190830073423.GB2354@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/7dedfd22b79822b7f4210e6255b672ea82db6678

Modified Files
--------------
src/include/common/int.h | 242 +++++++++++++++++++++++++++++++++++++++--------
1 file changed, 203 insertions(+), 39 deletions(-)