Typing: Don't restrict Prelude imports so much.

This code gets in the way if I start using (.). I find that
questionable, especially without tool support for maintaining this
import list.
Split checkLamBodyHasSort out of typecheckPull

I'm splitting this out by analogy with normalizeToSort, which does a
similar job. It seemed odd to call an error formatting function directly
inside typecheckPull.
normalizeToSort ↦ checkTypeOfIsSort

* This function does not do normalization any more.
* Moreover, this name is more coherent with checkLamBodyHasSort.
normalizeToSame ↦ checkIsEquiv

As above.
normalizeToPi ↦ checkIsPi