Isolate checksum calculation to its own module, so that bufpage
knows little if anything about the details of the calculation.

This implementation is a modified FNV-1a hash checksum, details
of which are given in the new checksum.c header comments.

Basic implementation only, so we fix the output value.

Later related commits will add version numbers to pg_control,
compiler optimization flags and memory barriers.

Ants Aasma, reviewed by Jeff Davis and Simon Riggs