Formal definition of cached call-by-name (incremental evaluation)

[yannham]

The incremental evaluation approach described in #1589 is based on what we dub "cached call-by-name", a form of very generic caching strategy which aims at including lazy evaluation, maximal lazyness, function memoization and incremental caching as particular instances.

While we shouldn't block on this issue to start experimenting (and we haven't - see #1649), it's important to have a better formalized definition of the cached call-by-name approach at some point.