Use `u64` for `required_unknown_bits_from` indexes, not `usize`

Use `u64` for `required_unknown_bits_from` indexes, not `usize`

While `usize` should be fine, we're multiplying the index by 8 so
if we have a jumbo feature bit fitting in a 32-bit size type may
not quite work. More importantly, this would be the first use of a
`usize` in the public API and dealing with it in bindings is
annoying so we just replace with a `u64`.