feat(features): remove FeatureBits bounds that are not elaborated and seal FeatureBits

[mkroening]

When users wanted to be generic over FeatureBits before, they had to duplicate the list of non-supertrait where bounds because only supertrait bounds are elaborated:

fn foo<T: FeatureBits>(features: T)
where
    virtio::F: From<T> + AsRef<T> + AsMut<T>;

This PR removes the not-elaborated where clauses to make generic code less verbose:

fn foo<T: FeatureBits>(features: T);

Of course, if the generic function wants to use some non-elaborated bounds, that is still possible.

cargo-semver-checks detects this as a breaking change, but I don't think it actually is. I think it does not handle the fact that Trait: Supertrait is equivalent to Trait where Self: Supertrait.

Still, we need to do a breaking release anyway because of #18, so this PR also seals FeatureBits. I don't think anyone would want to implement FeatureBits outside this crate without the internal helper macros anyway.