Bitfield should have an `is_disjoint` method

[michaelsproul]

To check whether two bitfields are disjoint you can use x.intersection(&y).is_empty(), however this allocates a new bitfield (vec-backed). If we can do this calculation without allocating it might improve attestation packing performance in Lighthouse, as this combination of calls is used extensively here:

https://github.com/sigp/lighthouse/blob/441fc1691b69f9edc4bbdc6665f3efab16265c9b/beacon_node/operation_pool/src/attestation_storage.rs#L128-L132