Add consensus::deserialize_hex function

We have `serialize_hex` and `deserialize` but no `deserialize_hex`, add
it.

bitcoin/src/consensus/mod.rs

@@ -11,6 +11,11 @@ pub mod params;
 #[cfg(feature = "bitcoinconsensus")]
 pub mod validation;
 
+use core::fmt;
+
+use hex::{FromHex, HexToBytesError};
+use internals::write_err;
+
 pub use self::encode::{
     deserialize, deserialize_partial, serialize, Decodable, Encodable, ReadExt, WriteExt,
 };
@@ -20,5 +25,50 @@ pub use self::validation::{
     verify_script, verify_script_with_flags, verify_transaction, verify_transaction_with_flags,
 };
 
+/// Deserialize any decodable type from a hex string.
+pub fn deserialize_hex<T: Decodable>(hex: &str) -> Result<T, FromHexError> {
+    let v = Vec::from_hex(hex)?;
+    Ok(deserialize(&v)?)
+}
+
+/// Hex decoding error.
+#[derive(Debug)]
+pub enum FromHexError {
+    /// Hex decoding error.
+    Decode(HexToBytesError),
+    /// Consensus deserialization error.
+    Deser(encode::Error),
+}
+
+impl fmt::Display for FromHexError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use FromHexError::*;
+
+        match *self {
+            Decode(ref e) => write_err!(f, "hex decoding erorr"; e),
+            Deser(ref e) => write_err!(f, "consensus deserialization error"; e),
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for FromHexError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use FromHexError::*;
+
+        match *self {
+            Decode(_) | Deser(_) => None,
+        }
+    }
+}
+
+impl From<HexToBytesError> for FromHexError {
+    fn from(e: HexToBytesError) -> Self { Self::Decode(e) }
+}
+
+impl From<encode::Error> for FromHexError {
+    fn from(e: encode::Error) -> Self { Self::Deser(e) }
+}
+
 #[cfg(feature = "serde")]
 pub mod serde;