Reverting to C++ style, and some other changes

Cargo.toml

@@ -62,6 +62,7 @@ rust-interpreter = []
 
 [dependencies]
 bitflags = "2.5"
+enum_primitive = "0.1"
 log = "0.4"
 ripemd = "0.1"
 secp256k1 = "0.29"

src/external/pubkey.rs

@@ -46,6 +46,13 @@ impl PubKey<'_> {
     }
 
     pub fn check_low_s(vch_sig: &Vec<u8>) -> bool {
-        todo!()
+        /* Zcash, unlike Bitcoin, has always enforced strict DER signatures. */
+        if let Ok(sig) = ecdsa::Signature::from_der(vch_sig) {
+            let mut check = sig.clone();
+            check.normalize_s();
+            sig == check
+        } else {
+            false
+        }
     }
 }