Cache inlined and categorified expressions

[zliu41]

Once we inline a variable and categorify its unfolding, we should cache it so that the next time we see it, we don't need to re-do the categorification work.

A/C: implement caching, benchmark it and the determine whether it should be adopted.

(Extricated from https://kitty-hawk.atlassian.net/browse/SW-3892)