Remove the assumption at the start of is_useful that any suitably-long array of
wildcard patterns is useful relative the any empty vector. Instead we just
continue to recurse column-wise over the matrix.

This assumption is false in the presence of empty types.
eg. in the simplest case:

let x: ! = ...;
match x {
    // This pattern should not be considered useful by the algorithm
    _   => ...
}

Fix is_uninhabited for enum types. It used to assume that an enums variant's
fields were all private.

Fix MIR generation for irrefutable Variant pattern matches. This allows code
like this to work:

    let x: Result<32, !> = Ok(123);
    let Ok(y) = x;

Carry type information on dummy wildcard patterns. Sometimes we need to expand
these patterns into their constructors and we don't want to be expanding a
TyError into a Constructor::Single.

Changes the non-exhaustive match error message to generate more general
witnesses.

You can now call .drain(..) on SmallVec, AccumulateVec and ArrayVec

We now cache the inhabitedness of types in the GlobalCtxt.

Rather than calculating whether a type is visibly uninhabited from a given
NodeId we calculate the full set of NodeIds from which a type is visibly
uninhabited then cache that set. We can then use that to answer queries about
the inhabitedness of a type relative to any given node.

Mostly just rename stuff.
Visibility checks use DefIds rather than NodeIds now.