Enhance the core transformation to strictify let pattern matching

[facundominguez]

This PR makes more effective a transformation in the code to rewrite

let pat = e0 in e1

to

case e0 of
  pat = e1

A new case is handled, and the transformation is constrained to apply only in cases where it doesn't break the type of the transformed expression.

Much of the challenge is that when we get to transform core, the lazy patterns have already been desugared to a bunch of lets which use projections instead. See the documentation of strictifyLazyLets for details.

The Isort.hs test had to be disabled as explained in #2335.

This PR is related to #2257. It doesn't address that issue, but it makes more predictable how LH will transform core. Fixing #2257 would require documenting the transformation to the user.