The Castable class is used by utils/. This Cl moves the implementation
into the utils/ folder. The `Is` and `As` methods are added into the
`tint` namespace to make usage shorter.
Change-Id: I0decedb92ebed01b6aa12d2e3efa7190742e9a33
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/127402
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>