Add From<Opcode> for u8 and roundtrip test

diff --git a/primitives/src/opcodes.rs b/primitives/src/opcodes.rs
index 19b94633..4f676804 100644
--- a/primitives/src/opcodes.rs
+++ b/primitives/src/opcodes.rs
@@ -42,6 +42,11 @@ impl From<u8> for Opcode {
     fn from(b: u8) -> Self { Self::from_u8(b) }
 }
 
+impl From<Opcode> for u8 {
+    #[inline]
+    fn from(op: Opcode) -> Self { op.to_u8() }
+}
+
 macro_rules! all_opcodes {
     ($($op:ident => $val:expr, $doc:expr);* $(;)?) => {
         /// Enables wildcard imports to bring into scope all opcodes and nothing else.
@@ -257,3 +262,19 @@ pub(crate) fn fmt_opcode(op: u8, f: &mut fmt::Formatter) -> fmt::Result {
         0xff => f.write_str("OP_INVALIDOPCODE"),
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn opcode_to_u8_from_u8_roundtrip() {
+        for b in 0..=u8::MAX {
+            let op = Opcode::from_u8(b);
+            assert_eq!(op.to_u8(), b);
+
+            let op = Opcode::from(b);
+            assert_eq!(u8::from(op), b);
+        }
+    }
+}