Add memoization to Envelopes

[byorgey]

Currently, every time an Envelope is queried it has to do O(n) work (where n is the number of primitives) since it has to evaluate a tree of n-1 applications of max. Envelopes ought to be memoized to cut down on the asymptotic time needed to e.g. lay out a list of diagrams using cat.

[bergey]

@Mathnerd314 If you want to tackle this, take a look at how Transformation is memoized using MemoTrie.