Cleanup: removed unused modules.

* PSO: pure Haskell PSO is so slow, it's useless
* Memo: pure Haskell memoization apparently has way too many cache
  misses compared to LRU cache written in C, although I don't know
  why, because the Haskell memoization doesn't forget anything while
  the LRU cache obviously does. The only relevant difference appears
  to be that the Haskell memoization used StableNames while the LRU
  cache uses StablePtrs.
* ExtFrac: typing this module was a lot of work, but I guess it's
  useless anyway, so let's drop it until it may be useful again.