fix(eddsa-poseidon): add mod reduction of secret scalar

The secret scalar is used to derive the public key and it must be > l, where l is the sub order used
in baby jubjub. Adding a mod reduction ensures it always stays in the right range by keeping it
compatible with the existing public keys.

re #239

packages/eddsa-poseidon/src/eddsa-poseidon.ts

@@ -53,7 +53,7 @@ export function deriveSecretScalar(privateKey: Buffer | Uint8Array | string): bi
     hash = hash.slice(0, 32)
     hash = pruneBuffer(hash)
 
-    return scalar.shiftRight(leBufferToBigInt(hash), BigInt(3))
+    return scalar.shiftRight(leBufferToBigInt(hash), BigInt(3)) % subOrder
 }
 
 /**