Use derive_const in nightly feature

[DaniPopes]

Feature nightly is currently broken:

error: expected a trait, found type
   --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustc-hash-2.1.2/src/lib.rs:110:14
    |
110 |         impl const Default for FxHasher {
    |              ^^^^^^^^^^^^^
...
119 | default_impl!();
    | --------------- in this macro invocation
    |
    = note: this error originates in the macro `default_impl` (in Nightly builds, run with -Z macro-backtrace for more info)

Fix this by using derive_const, and also a drive-by clippy fix.