Unused Rust code warnings

Recent Rust versions (1.26.1) report the following warnings on master:

warning: constant item is never used: `N_DIGEST_ALGORITHMS`
  --> external/crypto_digest.rs:74:1
   |
74 | const N_DIGEST_ALGORITHMS: usize = DIGEST_SHA3_512 as usize + 1;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(dead_code)] on by default

warning: type alias is never used: `smartlist_t`
   --> external/crypto_digest.rs:120:1
    |
120 | type smartlist_t = Stringlist;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^