Remove bits* typedefs.

Remove bits* typedefs.

In addition to removing the bits8, bits16, and bits32 typedefs,
this commit replaces all uses with uint8, uint16, or uint32.  bits*
provided little benefit beyond establishing the intent of the
variable, and they were inconsistently used for that purpose.
Third-party code should instead use the corresponding uint*
typedef.

Suggested-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://postgr.es/m/absbX33E4eaA0Ity%40nathan