Disallow ZSTs in `ThinMap`.

From thin-vec v0.2.20, `ThinVec` contains an optimization for the case
where the contained type is a ZST. In this case, the `ThinVec` will
avoid allocating any memory and simply store the length of the vector
plus one in the `ptr` field.

This means that the least significant bit of `ThinVec.ptr` may be 1 when
the contained type is a ZST. This is incompatible with the pointer tagging
mechanism of `ThinMap`, which uses the LSB of `ThinMap.ptr` to indicate whether
`ptr` is a transmuted `ThinVec` or a boxed map. The assumption is that a
transmuted `ThinVec` always has a LSB of 0, but this is not the case if
the `ThinVec` contains a ZST, and this will cause crashes in this case.

Of course, no one would construct a `ThinMap<(), ()>` explicitly, but
it's possible that this case could happen implicitly in a generic context, so
we should guard against it. (I assume the reason that `ThinVec`
optimizes for ZSTs in the first place is also because `ThinVec<()>` may
occur in generic contexts.)

Note that this disallows only the case where both the key and the value
are ZSTs. It is still permissible for either the key or the value to be
ZSTs (and the latter case in particular is important because it is used
by `ThinSet`).