mkWWargs's job was pushing casts inwards and doing eta expansion to match
the arity with the number of argument demands we w/w for.
Nowadays, we use the Simplifier to eta expand to arity. In fact, in recent years we have even seen the eta expansion done by w/w as harmful, see Note [Don't eta expand in w/w]. If a function hasn't enough manifest lambdas, don't w/w it!
What purpose does mkWWargs serve in this world? Not a great one, it turns out!
I could remove it by pulling some important bits,
notably Note [Freshen WW arguments] and Note [Join points and beta-redexes].
Result: We reuse the freshened binder names of the wrapper in the
worker where possible (see testuite changes), much nicer!
In order to avoid scoping errors due to lambda-bound unfoldings in worker arguments, we zap those unfoldings now. In doing so, we fix #19766 (closed).
Fixes #19874 (closed).