Move pg_crc.c to src/common, and remove pg_crc_tables.h

To get CRC functionality in a client program, you now need to link with
libpgcommon instead of libpgport. The CRC code has nothing to do with
portability, so libpgcommon is a better home. (libpgcommon didn't exist
when pg_crc.c was originally moved to src/port.)

Remove the possibility to get CRC functionality by just #including
pg_crc_tables.h. I'm not aware of any extensions that actually did that and
couldn't simply link with libpgcommon.

This also moves the pg_crc.h header file from src/include/utils to
src/include/common, which will require changes to any external programs
that currently does #include "utils/pg_crc.h". That seems acceptable, as
include/common is clearly the right home for it now, and the change needed
to any such programs is trivial.

contrib/ltree/crc32.c

@@ -20,7 +20,7 @@
 #define TOLOWER(x)	(x)
 #endif
 
-#include "utils/pg_crc.h"
+#include "common/pg_crc.h"
 #include "crc32.h"
 
 unsigned int