Tag `KVStore` `(C-not exported)` as `Writeable` isn't mapped

Currently `Writeable` is mapped manually, making it impossible to
automatically map a trait method that is parameterized by
`Writeable` (as is true for the `write` method on `KVStore`).

Ultimately we'll want to move to automatically mapping `Writeable`
like any other trait (only manually mapping the std `Write` and
`Read` traits), so this is only a candidate for the bindings branch,
not upstream. That may take a few releases, however.