remove use of 'hashes' from public API

We had an impl of the (deprecated) ThirtyTwoByte hash trait, which we no
longer use; we delete this. We also had a `pub extern crate` which we
delete. Also remove the 'hashes' feature gate from a unit test that no
longer needs it.

Author: apoelstra

Cargo.toml

@@ -58,11 +58,11 @@ unexpected_cfgs = { level = "deny", check-cfg = ['cfg(bench)', 'cfg(secp256k1_fu
 
 [[example]]
 name = "sign_verify_recovery"
-required-features = ["recovery", "hashes", "std"]
+required-features = ["recovery", "std"]
 
 [[example]]
 name = "sign_verify"
-required-features = ["hashes", "std"]
+required-features = ["std"]
 
 [[example]]
 name = "generate_keys"

src/key.rs

@@ -16,9 +16,6 @@ use crate::ellswift::ElligatorSwift;
 use crate::ffi::types::c_uint;
 use crate::ffi::{self, CPtr};
 use crate::Error::{self, InvalidPublicKey, InvalidPublicKeySum, InvalidSecretKey};
-#[cfg(feature = "hashes")]
-#[allow(deprecated)]
-use crate::ThirtyTwoByteHash;
 #[cfg(feature = "global-context")]
 use crate::SECP256K1;
 use crate::{
@@ -371,15 +368,6 @@ impl SecretKey {
     }
 }
 
-#[cfg(feature = "hashes")]
-#[allow(deprecated)]
-impl<T: ThirtyTwoByteHash> From<T> for SecretKey {
-    /// Converts a 32-byte hash directly to a secret key without error paths.
-    fn from(t: T) -> SecretKey {
-        SecretKey::from_byte_array(t.into_32()).expect("failed to create secret key")
-    }
-}
-
 #[cfg(feature = "serde")]
 impl serde::Serialize for SecretKey {
     fn serialize<S: serde::Serializer>(&self, s: S) -> Result<S::Ok, S::Error> {
@@ -1889,7 +1877,7 @@ mod test {
     }
 
     #[test]
-    #[cfg(all(feature = "rand", feature = "alloc", not(feature = "hashes")))]
+    #[cfg(all(feature = "rand", feature = "alloc"))]
     fn test_debug_output() {
         let s = Secp256k1::new();
         let (sk, _) = s.generate_keypair(&mut StepRng::new(1, 1));

src/lib.rs

@@ -152,9 +152,6 @@ extern crate core;
 #[cfg(bench)]
 extern crate test;
 
-#[cfg(feature = "hashes")]
-pub extern crate hashes;
-
 #[macro_use]
 mod macros;
 #[macro_use]