Update scrypt fuzz target for current API

[xingyaner]

Problem

The scrypt fuzz target no longer builds against the current scrypt API.
The target uses outdated password‑hash types and constructors, including SaltString, Ident, and the previous Params::new interface. It also enables the obsolete simple feature.

As a result, the fuzz target cannot be built with the current dependency implementation.

Reproduction steps

1. Check out the affected password‑hashes revision.
2. Enter the fuzz directory.
3. Run:

cargo fuzz build

4. Observe compilation errors related to the removed or changed scrypt API.

Expected behavior

The scrypt fuzz target should compile successfully against the current scrypt implementation and continue testing both direct scrypt derivation and PHC password hashing and verification.

Proposed solution

Update the fuzz target to use the current Params, Scrypt::new_with_params, and PHC APIs. Replace the obsolete simple feature with the required phc and alloc features.

[xingyaner]

The PR is #927 .