For the associated type `CustomizedPasswordHasher::Params`, removes the
`Display` and `FromStr` bounds which aren't actually used: for
PHC-related purposes which are the only ones implemented directly in
`password-hash`, it instead requires a `TryFrom<phc::ParamsString>` impl
instead.

Requiring these as well seemed like good hygiene so there's some way to
serialize and parse the params as a string, but it becomes a problem
when the params are shared between e.g. PHC and MCF implementations
where each of them has a different params serialization so there's not
one "true" serialization for which it actually makes sense to impl
`Display` and `FromStr`.