Be pickier about unsaturated synonyms in :kind

Summary:
We currently permit any and all uses of unsaturated type
synonyms and type families in GHCi's `:kind` command, which allows
strange interactions like this one:

```
> :set -XTypeFamilies -XPolyKinds
> type family Id (a :: k)
> type instance Id a = a
> type Foo x = Maybe
> :kind! Id Foo
```

This is probably a stretch too far, so this patch moves to disallow
unsaturated synonyms that aren't at the top level (we still want to
allow `:kind Id`, for instance). We do this by augmenting `GhciCtxt`
with an additional `Bool` field to indicate if we are at the
outermost level of the type being passed to `:kind` or not. See
`Note [Unsaturated type synonyms in GHCi]` in `TcValidity` for the
full story.

Test Plan: make test TEST=T16013

Reviewers: goldfire, bgamari

Reviewed By: goldfire

Subscribers: simonpj, goldfire, rwbarton, carter

GHC Trac Issues: #16013

Differential Revision: https://phabricator.haskell.org/D5471

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