normalizeToSort: simplify invocation

Remove the duplication among callers. Next step: rename it.
examples/neg/lambda.lpts: specify PTS instance

To specify the instance inline, we currently have to make files
executable and add the information in the shebang, waiting for a
solution for #100.
Complain correctly for sortless functions bodies

Fix #107.
Fix #108.

(Updated: Fixed typo in error msg).
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 ↦ checkProperType

* This function does not do normalization any more.
* Moreover, this name (suggested by Tillmann) is more coherent with both
  checkLamBodyHasSort and msgNotProperType.
normalizeToSame ↦ checkIsEquiv

As above.
normalizeToPi ↦ checkIsPi
Drop unused normalizeToInt

This completes the removal/rename of normalizeTo* functions.
Merge pull request #115 from Blaisorblade/topic/sort-checking2

Fix sort checking v2