Refactor memoized helpers

Created by: alindeman

This is a cleanup of #871 that @JonRowe and I worked on.

It fixes #908 and #817 (closed) and is references in (rspec/rspec-rails#738)

We're not 100% thrilled with it, but we could not think of a better way to fix the relevant issues while still allowing super within let.

Thoughts?