WORK TOWARDS HIERARCHICAL TEMPLATE IN DISCUSSION

I was there, had it working by some weird hackish
method.  Didn't commit.  Tried to rework things
properly, and then found I couldn't get back to
the place where I was before.

Fairly confused by all the type synonyms, but
feel like I'm making some progress.
(HeistT n n Template) is (Splice) is (SnapletISplice App)
(Splices m) is (Splices (SnapletISplice App))
roughly.

Avoiding type errors means not putting a singular in
a plural's place, wrt. Splice vs. Splices.