Speed up CRC calculation using slicing-by-8 algorithm.

 Speed up CRC calculation using slicing-by-8 algorithm.
 
 This speeds up WAL generation and replay. The new algorithm is
 significantly faster with large inputs, like full-page images or when
 inserting wide rows. It is slower with tiny inputs, i.e. less than 10 bytes
 or so, but the speedup with longer inputs more than make up for that. Even
 small WAL records at least have 24 byte header in the front.
 
 The output is identical to the current byte-at-a-time computation, so this
 does not affect compatibility. The new algorithm is only used for the
 CRC-32C variant, not the legacy version used in tsquery or the
 "traditional" CRC-32 used in hstore and ltree. Those are not as performance
 critical, and are usually only applied over small inputs, so it seems
 better to not carry around the extra lookup tables to speed up those rare
 cases.
 
 Abhijit Menon-Sen