Make candidateQTvs contain tyvar with zonked kinds
candidateQTyVars was failing to return fully-zonked
tyvars, and that made things fall over chaotically
when we try to sort them into a well-scoped telescope.
Result: Trac Trac #15795
So I made candidateQTvs guarantee to have fully-zonked
tyvars (i.e. with zonked kinds). That's a bit annoying
but not really difficult.