Fix bi-directional range content bindings

This requires additional coordination between the source to target and
target to source data flow.

To the point, I’m beginning to think that making `rangeContent()` an
operator might be better represented by a special `<=>` content two-way
binding operator. Certainly, this would obviate the need for
`rangeContent` operator algebra.
Merge pull request #1 from kriskowal/bidi-range-content-bindings

Fix bi-directional range content bindings