This leads to a lot of simplifications, as most code doesn't actually need to know about the specific lifetime/type data; rather, it's concerned with properties like name, index and def_id.

This makes it consistent with TypeParameterDef.

This clarifies the intent of whether to emit deprecated lint or not.

Types will no longer default to `()`, instead always defaulting to `!`.
This disables the associated warning and removes the flag from TyTuple

This adds an `UnpackedKind` type as a typesafe counterpart to `Kind`. This should make future changes to kinds (such as const generics!) more resilient, as the type-checker should catch more potential issues.

a default impl should never be considered as implementing the
trait on its own -- regardless of whether it contains all items
or not

    a `default impl` need not include all items from the trait
    a `default impl` alone does not mean that a type implements the trait

It turns out that this *can* happen after all, if the region is only
used in projections from the input types.