Pass byte slice to to_hex (#3307)

Currently we pass a Vec. This requires an extra allocation and copy of all elements if a caller doesn't have a Vec already, which is at least 95% of cases.
Another, a smaller issue, we have a function util::to_hex and some structs implement to_hex() on top of it, so we have a mix of it in the code. This PR introduces a trait and a blanket impl for AsRef<[u8]> which brings a uniform API (obj.to_hex()). One unfortunate case is arrays of size bigger than 32 - Rust doesn't implement AsRef for them so it requires an ugly hack (&array[..]).to_hex().

keychain/src/mnemonic.rs

@@ -172,7 +172,7 @@ where
 #[cfg(test)]
 mod tests {
 	use super::{from_entropy, to_entropy, to_seed};
-	use crate::util::{from_hex, to_hex};
+	use crate::util::{from_hex, ToHex};
 	use rand::{thread_rng, Rng};
 
 	struct Test<'a> {
@@ -311,7 +311,7 @@ mod tests {
 		let tests = tests();
 		for t in tests.iter() {
 			assert_eq!(
-				to_hex(to_seed(t.mnemonic, "TREZOR").unwrap().to_vec()),
+				(&to_seed(t.mnemonic, "TREZOR").unwrap()[..]).to_hex(),
 				t.seed.to_string()
 			);
 			assert_eq!(

keychain/src/types.rs

@@ -27,12 +27,12 @@ use crate::blake2::blake2b::blake2b;
 use crate::extkey_bip32::{self, ChildNumber};
 use serde::{de, ser}; //TODO: Convert errors to use ErrorKind
 
-use crate::util;
 use crate::util::secp::constants::SECRET_KEY_SIZE;
 use crate::util::secp::key::{PublicKey, SecretKey};
 use crate::util::secp::pedersen::Commitment;
 use crate::util::secp::{self, Message, Secp256k1, Signature};
 use crate::util::static_secp_instance;
+use crate::util::ToHex;
 use zeroize::Zeroize;
 
 use byteorder::{BigEndian, ReadBytesExt, WriteBytesExt};
@@ -195,10 +195,6 @@ impl Identifier {
 		Ok(Identifier::from_bytes(&bytes))
 	}
 
-	pub fn to_hex(&self) -> String {
-		util::to_hex(self.0.to_vec())
-	}
-
 	pub fn to_bip_32_string(&self) -> String {
 		let p = ExtKeychainPath::from_identifier(&self);
 		let mut retval = String::from("m");
@@ -288,10 +284,6 @@ impl BlindingFactor {
 		BlindingFactor::from_secret_key(secp::key::ZERO_KEY)
 	}
 
-	pub fn to_hex(&self) -> String {
-		util::to_hex(self.0.to_vec())
-	}
-
 	pub fn from_hex(hex: &str) -> Result<BlindingFactor, Error> {
 		let bytes = util::from_hex(hex).unwrap();
 		Ok(BlindingFactor::from_slice(&bytes))