Fix comparing collections with filters

When comparing two collections with a compare_filter, the 'id_args' method
would use an empty field selector to subscript the compare_filter.  This
should be an identity operation but isn't currently, so work around it by
only subscripting by that filter if it is non-zero length.
Give MultiFieldSelector subscript operation an identity value

This seems the most correct thing to do in this situation, and makes some
usages work naturally.  Extend this to the 'in' operator, and while we're
there, skip using collections.Sequence in that function, because it matches
strings.
Release engineering for 0.5.1 release