Issue #6690: Optimize the bytecode for expressions such as `x in {1, 2, 3}`,

where the right hand operand is a set of constants, by turning the set into
a frozenset and pre-building it as a constant.  The comparison operation
is made against the constant instead of building a new set each time it is
executed (a similar optimization already existed which turned a list of
constants into a pre-built tuple).  Patch and additional tests by Dave
Malcolm.