GenEx: generalize bound operands to a frame and add $<_1>

Allow a binding operation to bind more than one operand at once, exposed as
$<_0>, $<_1>, ....  A single value remains the common case, but the upcoming
SORT COMPARATOR must bind the two elements being compared, so the binding
becomes an indexed frame and $<_1> is added.  Referencing an index the active
binding does not provide is reported as an error.

Issue: #27892