Add support for ALTER RULE ... RENAME TO.

doc/src/sgml/ref/allfiles.sgml

doc/src/sgml/ref/alter_rule.sgml

doc/src/sgml/ref/create_rule.sgml

doc/src/sgml/ref/drop_rule.sgml

doc/src/sgml/reference.sgml

src/backend/commands/alter.c

src/backend/parser/gram.y

src/backend/rewrite/rewriteDefine.c

src/bin/psql/tab-complete.c

src/include/rewrite/rewriteDefine.h

src/test/regress/expected/rules.out

src/test/regress/sql/rules.sql