DDL support for collations

- collowner field
- CREATE COLLATION
- ALTER COLLATION
- DROP COLLATION
- COMMENT ON COLLATION
- integration with extensions
- pg_dump support for the above
- dependency management
- psql tab completion
- psql \dO command

doc/src/sgml/reference.sgml

@@ -35,6 +35,7 @@
 
    &abort;
    &alterAggregate;
+   &alterCollation;
    &alterConversion;
    &alterDatabase;
    &alterDefaultPrivileges;
@@ -76,6 +77,7 @@
    &copyTable;
    &createAggregate;
    &createCast;
+   &createCollation;
    &createConversion;
    &createDatabase;
    &createDomain;
@@ -113,6 +115,7 @@
    &do;
    &dropAggregate;
    &dropCast;
+   &dropCollation;
    &dropConversion;
    &dropDatabase;
    &dropDomain;