Bloom index contrib module

Module provides new access method. It is actually a simple Bloom filter
implemented as pgsql's index. It could give some benefits on search
with large number of columns.

Module is a single way to test generic WAL interface committed earlier.

Author: Teodor Sigaev, Alexander Korotkov
Reviewers: Aleksander Alekseev, Michael Paquier, Jim Nasby

doc/src/sgml/contrib.sgml

@@ -105,6 +105,7 @@ CREATE EXTENSION <replaceable>module_name</> FROM unpackaged;
  &adminpack;
  &auth-delay;
  &auto-explain;
+ &bloom;
  &btree-gin;
  &btree-gist;
  &chkpass;