…here, r=<try>

No StableHasherResult everywhere

This removes the generic parameter on `StableHasher`, instead moving it to the call to `finish`. This has the side-effect of making all `HashStable` impls nicer, since we no longer need the verbose `<W: StableHasherResult>` that previously existed -- often forcing line wrapping.

This is done for two reasons:
 * we should avoid false "generic" dependency on the result of StableHasher
     * we don't need to codegen two/three copies of all the HashStable impls when they're transitively used to produce a fingerprint, u64, or u128. I haven't measured, but this might actually make our artifacts somewhat smaller too.
 * Easier to understand/read/write code -- the result of the stable hasher is irrelevant when writing a hash impl.

cc @eddyb @nikomatsakis I guess? Not sure whose purview this falls under

r? @michaelwoerister due to touching incremental

… needs to take that into account.