WIP caching semantics for CBPV terms

Boring problems appear with constants. Interesting questions appear with
the "monadic" bind and return of CBPV: if we can satisfy monad laws,
some things get much simpler.
WIP sketch System L
Readd UNDEFINED

This was deleted in 5bde6429aaee189aecc700b69ff698946d226090: undefined
is not appropriate for a complete development, but it is for a
work-in-progress (like the current one).
Use UNDEFINED to fix "unsolved metas" errors

These errors were introduced in the recent WIP commits.