Add SP-GiST (space-partitioned GiST) index access method.

SP-GiST is comparable to GiST in flexibility, but supports non-balanced
partitioned search structures rather than balanced trees.  As described at
PGCon 2011, this new indexing structure can beat GiST in both index build
time and query speed for search problems that it is well matched to.

There are a number of areas that could still use improvement, but at this
point the code seems committable.

Teodor Sigaev and Oleg Bartunov, with considerable revisions by Tom Lane

doc/src/sgml/ref/create_opclass.sgml

@@ -172,7 +172,7 @@ CREATE OPERATOR CLASS <replaceable class="parameter">name</replaceable> [ DEFAUL
       the input data type(s) of the function (for B-tree comparison functions
       and hash functions)
       or the class's data type (for B-tree sort support functions and all
-      functions in GiST and GIN operator classes).  These defaults
+      functions in GiST, SP-GiST and GIN operator classes).  These defaults
       are correct, and so <replaceable
       class="parameter">op_type</replaceable> need not be specified in
       <literal>FUNCTION</> clauses, except for the case of a B-tree sort
@@ -232,7 +232,7 @@ CREATE OPERATOR CLASS <replaceable class="parameter">name</replaceable> [ DEFAUL
      <para>
       The data type actually stored in the index.  Normally this is
       the same as the column data type, but some index methods
-      (currently GIN and GiST) allow it to be different.  The
+      (currently GiST and GIN) allow it to be different.  The
       <literal>STORAGE</> clause must be omitted unless the index
       method allows a different type to be used.
      </para>