Add a secp256k1_i128_to_u64 function.

src/int128_native_impl.h

@@ -67,7 +67,12 @@ static SECP256K1_INLINE void secp256k1_i128_rshift(secp256k1_int128 *r, unsigned
    *r >>= n;
 }
 
+static SECP256K1_INLINE uint64_t secp256k1_i128_to_u64(const secp256k1_int128 *a) {
+   return (uint64_t)*a;
+}
+
 static SECP256K1_INLINE int64_t secp256k1_i128_to_i64(const secp256k1_int128 *a) {
+   VERIFY_CHECK(INT64_MIN <= *a && *a <= INT64_MAX);
    return *a;
 }