Tidy up join point binders in the simplifier
When looking at something else I discovered that join-point binders were being fiddled
- via
setJoinResTy
inSimplify.Env.substNonCoVarIdBndr
- via
scaleJoinPointType
inSimplify.simplJoinBind
Not only is this untidy, but the latter doesn't update the in-scope set and/or substitution, which might be bad.