Concretize `WriteableScore` into `MultiThreadedLockableScore`

Concretize `WriteableScore` into `MultiThreadedLockableScore`

In general the bindings don't handle blanket implementations well -
they generate concrete implementations for everything and don't
bother building up enough context to be aware of the blanket
implementation to avoid duplicating it while still allowing users
to access struct(s) as all implemented traits.

Thus, implementing `WriteableScore` for all `LockableScore`s that
also implement `Writeable` is particularly impractical to map in
bindings.

Further, because `Score` already requires `Writeable`, having a
separate `WriteableScore` doesn't really make any sense.

Here we simply remove `WriteableScore` (in `c_bindings` mode)
entirely and push users through `MultiThreadedLockableScore` in the
higher-level traits that require `Score`.