Note [Making dictionary parameters strict]

The Opt_DictsStrict flag makes GHC use call-by-value for dictionaries.  Why?

* Generally CBV is more efficient.

* A datatype dictionary is always non-bottom and never takes much work to
  compute.  E.g. a DFun from an instance decl always returns a dictionary
  record immediately.  See DFunUnfolding in CoreSyn.
  See also Note [Recursive superclasses] in TcInstDcls.

See #17758 for more background and perf numbers.

Wrinkles:

* A newtype dictionary is *not* always non-bottom.  E.g.
      class C a where op :: a -> a
      instance C Int where op = error "urk"
  Now a value of type (C Int) is just a newtype wrapper (a cast) around
  the error thunk.  Don't strictify these!

* Strictifying DFuns risks destroying the invariant that DFuns never take much
  work to compute, so we don't do it.
  See Note [Do not strictify a DFun's parameter dictionaries] for details.

* Although worker/wrapper *could* unbox strictly used dictionaries, we do not do
  so; see Note [Do not unbox class dictionaries].

The implementation is extremely simple: just make the strictness
analyser strictify the demand on a dictionary binder in
'findBndrDmd' if the binder does not belong to a DFun.