Add mbverifystr() functions specific to each encoding.

 Add mbverifystr() functions specific to each encoding.
 
 This makes pg_verify_mbstr() function faster, by allowing more efficient
 encoding-specific implementations. All the implementations included in
 this commit are pretty naive, they just call the same encoding-specific
 verifychar functions that were used previously, but that already gives a
 performance boost because the tight character-at-a-time loop is simpler.
 
 Reviewed-by: John Naylor
 Discussion: https://www.postgresql.org/message-id/e7861509-3960-538a-9025-b75a61188e01@iki.fi