Rename contains/contained-by operators to @> and <@, per discussion that

Rename contains/contained-by operators to @> and <@, per discussion that
agreed these symbols are less easily confused.  I made new pg_operator
entries (with new OIDs) for the old names, so as to provide backward
compatibility while making it pretty easy to remove the old names in
some future release cycle.  This commit only touches the core datatypes,
contrib will be fixed separately.