This patch was triggered by Trac #11601, where I discovered that
-XStrict was really not doing the right thing. In particular,

  f y = let !(Just x) = blah[y] in body[y,x]

This was evaluating 'blah' but not pattern matching it
against Just until x was demanded.  This is wrong.

The patch implements a new semantics which ensures that strict
patterns (i.e. ones with an explicit bang, or with -XStrict)
are evaluated fully when bound.

* There are extensive notes in DsUtils:
  Note [mkSelectorBinds]

* To do this I found I need one-tuples;
  see Note [One-tuples] in TysWiredIn

I updated the user manual to give the new semantics