document the zeroize Cargo feature

As part of this change, I don't think we need the `zeroize_crate`
workaround anymore if we use the relateively new `dep:` syntax in
Cargo.toml.

Cargo.toml

@@ -28,6 +28,10 @@ std = []
 # The "rayon" feature (defined below as an optional dependency) enables the
 # `Hasher::update_rayon` method, for multithreaded hashing. However, even if
 # this feature is enabled, all other APIs remain single-threaded.
+rayon = ["dep:rayon", "std"]
+
+# Implement the zeroize::Zeroize trait for types in this crate.
+zeroize = ["dep:zeroize", "arrayvec/zeroize"]
 
 # This crate implements traits from the RustCrypto project, exposed here as the
 # "traits-preview" feature. However, these traits aren't stable, and they're
@@ -77,11 +81,9 @@ no_avx2 = []
 no_avx512 = []
 no_neon = []
 
-zeroize = ["zeroize_crate", "arrayvec/zeroize"]
-
 [package.metadata.docs.rs]
 # Document Hasher::update_rayon on docs.rs.
-features = ["rayon"]
+features = ["rayon", "zeroize"]
 
 [dependencies]
 arrayref = "0.3.5"
@@ -90,7 +92,7 @@ constant_time_eq = "0.3.0"
 rayon = { version = "1.2.1", optional = true }
 cfg-if = "1.0.0"
 digest = { version = "0.10.1", features = [ "mac" ], optional = true }
-zeroize_crate = { package = "zeroize", version = "1", default-features = false, features = ["zeroize_derive"], optional = true }
+zeroize = { version = "1", default-features = false, features = ["zeroize_derive"], optional = true }
 
 [dev-dependencies]
 hex = "0.4.2"

src/lib.rs

@@ -43,6 +43,9 @@
 //! the [`Hasher::update_rayon`] method, for multithreaded hashing. However,
 //! even if this feature is enabled, all other APIs remain single-threaded.
 //!
+//! The `zeroize` feature (disabled by default, but enabled for [docs.rs]) implements
+//! [`Zeroize`](https://docs.rs/zeroize/latest/zeroize/trait.Zeroize.html) for this crate's types.
+//!
 //! The NEON implementation is enabled by default for AArch64 but requires the
 //! `neon` feature for other ARM targets. Not all ARMv7 CPUs support NEON, and
 //! enabling this feature will produce a binary that's not portable to CPUs
@@ -67,10 +70,6 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
-#[cfg(feature = "zeroize")]
-extern crate zeroize_crate as zeroize; // Needed because `zeroize::Zeroize` assumes the crate is named `zeroize`.
-
-
 #[cfg(test)]
 mod test;