Add link to `Elegant memoization` from Conal Elliott

[safareli]

Once I saw implementation of this lib Elegant memoization came to my mind. and thought that it might be good for curious reader to take a look at the slides.

[safareli]

It also has laws, which could be adopted

trie (untrie a) = a
untrie (trie f ) = f

[paf31]

Well this library doesn't use an intermediate trie data type, at least not one which is visible in the API. The closest library would probably be data-memocombinators. That said, I wouldn't be against adding links to relevant papers/libraries.