Stop indexing hash types

In preparation for removing `SliceIndex` from hash type impls (in
`bitcoin_hashes`) lets stop indexing hash types here.

(Done mainly because I can't remember right now how to get secp to stop
using local version of hashes when developing patches in
`rust-bitcoin` and it needs doing eventually anyways.)

src/secret.rs

@@ -20,8 +20,8 @@ macro_rules! impl_display_secret {
 
                 let mut engine = sha256::Hash::engine();
                 let tag_hash = sha256::Hash::hash(tag.as_bytes());
-                engine.input(&tag_hash[..]);
-                engine.input(&tag_hash[..]);
+                engine.input(&tag_hash.as_ref());
+                engine.input(&tag_hash.as_ref());
                 engine.input(&self.secret_bytes());
                 let hash = sha256::Hash::from_engine(engine);