Eliminate unnecessary conversion from cursor steps to range
Remove unnecessary (and impossible to use) constraint argument

The constraint function only allows a choice between two adjacent steps
if the supplied position is not exactly on a step. With things such as
inline roots and other complex features of the addressing system, this
is extremely difficult to use correctly.

As it is currently unused, the best thing to prevent future problems it
to terminate it now!
Split CollapsingRules out of OpRemoveText

This functionality will be shared with the soon-to-exist merge paragraph
operation.
WIP merge paragraph