Considering using ssri crate for subresource integrity parsing

[Manishearth]

We already support parsing and checking subresource integrity headers in https://github.com/servo/servo/blob/master/components/net/subresource_integrity.rs, but there is the ssri crate which is being actively developed. Maybe we should consider pulling that in?

As an aside, we should also consider splitting out such algorithms into external crates more often.

[jgraham]

Note that the license of the 5.0.0 release is unusual ("Freedom Zero Parity") and so switching to this would probably require either forking at 4.x or a specific determination that this license is acceptable to servo (unless that already happened ofc).