Implement multivariate n-distinct coefficients

Add support for explicitly declared statistic objects (CREATE
STATISTICS), allowing collection of statistics on more complex
combinations that individual table columns.  Companion commands DROP
STATISTICS and ALTER STATISTICS ... OWNER TO / SET SCHEMA / RENAME are
added too.  All this DDL has been designed so that more statistic types
can be added later on, such as multivariate most-common-values and
multivariate histograms between columns of a single table, leaving room
for permitting columns on multiple tables, too, as well as expressions.

This commit only adds support for collection of n-distinct coefficient
on user-specified sets of columns in a single table.  This is useful to
estimate number of distinct groups in GROUP BY and DISTINCT clauses;
estimation errors there can cause over-allocation of memory in hashed
aggregates, for instance, so it's a worthwhile problem to solve.  A new
special pseudo-type pg_ndistinct is used.

(num-distinct estimation was deemed sufficiently useful by itself that
this is worthwhile even if no further statistic types are added
immediately; so much so that another version of essentially the same
functionality was submitted by Kyotaro Horiguchi:
https://postgr.es/m/20150828.173334.114731693.horiguchi.kyotaro@lab.ntt.co.jp
though this commit does not use that code.)

Author: Tomas Vondra.  Some code rework by Álvaro.
Reviewed-by: Dean Rasheed, David Rowley, Kyotaro Horiguchi, Jeff Janes,
    Ideriha Takeshi
Discussion: https://postgr.es/m/543AFA15.4080608@fuzzy.cz
    https://postgr.es/m/20170320190220.ixlaueanxegqd5gr@alvherre.pgsql

src/include/nodes/relation.h

@@ -529,6 +529,7 @@ typedef struct RelOptInfo
 	List	   *lateral_vars;	/* LATERAL Vars and PHVs referenced by rel */
 	Relids		lateral_referencers;	/* rels that reference me laterally */
 	List	   *indexlist;		/* list of IndexOptInfo */
+	List	   *statlist;		/* list of StatisticExtInfo */
 	BlockNumber pages;			/* size estimates derived from pg_class */
 	double		tuples;
 	double		allvisfrac;
@@ -668,6 +669,24 @@ typedef struct ForeignKeyOptInfo
 	List	   *rinfos[INDEX_MAX_KEYS];
 } ForeignKeyOptInfo;
 
+/*
+ * StatisticExtInfo
+ *		Information about extended statistics for planning/optimization
+ *
+ * This contains information about which columns are covered by the
+ * statistics (stakeys), which options were requested while adding the
+ * statistics (*_enabled), and which kinds of statistics were actually
+ * built and are available for the optimizer (*_built).
+ */
+typedef struct StatisticExtInfo
+{
+	NodeTag		type;
+
+	Oid			statOid;		/* OID of the statistics row */
+	RelOptInfo *rel;			/* back-link to index's table */
+	char		kind;			/* statistic kind of this entry */
+	Bitmapset  *keys;			/* attnums of the columns covered */
+} StatisticExtInfo;
 
 /*
  * EquivalenceClasses