`thir::Pat::kind` is a `Box<PatKind>`, which doesn't follow the usual
pattern in AST/HIR/THIR which is that the "kind" enum for a node is
stored inline within the parent struct.
This commit makes the `PatKind` directly inline within the `Pat`. This
requires using `Box<Pat>` in all the types that hold a `Pat.
Ideally, `Pat` would be stored in `Thir` like `Expr` and `Stmt` and
referred to with a `PatId` rather than `Box<Pat>`. But this is hard to
do because lots of `Pat`s get created after the destruction of the `Cx`
that does normal THIR building. But this does get us a step closer to
`PatId`, because all the `Box<Pat>` occurrences would be replaced with
`PatId` if `PatId` ever happened.
At 128 bytes, `Pat` is large. Subsequent commits will shrink it.
