Add new `frame` API (more general than `split`/`msplit` stuff).  This is essential to preserving intercalary "label"/context text which, in turn, can be essential for end users to interpret substitutional transformations (e.g. replacing numbers with their means over lines).
Add new `frame` API (more general than `split`/`msplit` stuff).  This is
essential to preserving intercalary "label"/context text which, in turn, can
be essential for end users to interpret substitutional transformations (e.g.
replacing numbers with their means over lines).

Arguably, the split APIs should all be re-written in terms of `frame`, but
that is a battle for another day.

c-blake/cligen