Note [Kind generalisation]

Step 5 of Note [Recipe for checking a signature], namely
kind-generalisation, is done by
    kindGeneraliseAll
    kindGeneraliseSome
    kindGeneraliseNone

Here, we have to deal with the fact that metatyvars generated in the
type will have a bumped TcLevel, because explicit foralls raise the
TcLevel. To avoid these variables from ever being visible in the
surrounding context, we must obey the following dictum:

  Every metavariable in a type must either be
    (A) generalized, or
    (B) promoted, or        See Note [Promotion in signatures]
    (C) a cause to error    See Note [Naughty quantification candidates]
                            in GHC.Tc.Utils.TcMType

There are three steps (look at kindGeneraliseSome):

1. candidateQTyVarsOfType finds the free variables of the type or kind,
   to generalise

2. filterConstrainedCandidates filters out candidates that appear
   in the unsolved 'wanteds', and promotes the ones that get filtered out
   thereby.

3. quantifyTyVars quantifies the remaining type variables

The kindGeneralize functions do not require pre-zonking; they zonk as they
go.

kindGeneraliseAll specialises for the case where step (2) is vacuous.
kindGeneraliseNone specialises for the case where we do no quantification,
but we must still promote.

If you are actually doing kind-generalization, you need to bump the
level before generating constraints, as we will only generalize
variables with a TcLevel higher than the ambient one.
Hence the "pushLevel" in pushLevelAndSolveEqualities.