digest: add TryCustomizedInit trait

[newpavlov]

Some algorithms (e.g. Ascon-CXOF128 and bash-prg-hash) place restrictions on customization strings, so with the current version of digest we either have to panic on invalid strings or use inherent methods. This PR amends this by introducing a fallible variant of CustomizedInit with the blanket impl to act as a bridge between the traits.

[newpavlov]

Usually I would prefer to do this instead:

pub trait TryCustomizedInit: Sized {
    type Error;
    fn try_new_customized(customization: &[u8]) -> Result<Self, Self::Error>;
}

pub trait CustomizedInit: TryCustomizedInit<Error = Infallible> {
    fn new_customized(customization: &[u8]) -> Self {
        let Ok(res) = self.try_new_customized(customization);
        res
    }
}

impl<T: TryCustomizedInit<Error = Infallible> CustomizedInit for T {}

But it would be a breaking change, so it has to be deferred until a future breaking release.