Add power monoid for __gnu_cxx::__power

Author: celonymire

include/celony/miscellaneous/power_monoid.hpp

@@ -0,0 +1,8 @@
+template <int MOD = (int)1e9 + 7, typename T = long long> struct power_monoid {
+  constexpr T operator()(const T &a, const T &b) const noexcept {
+    return 1ll * a * b % MOD;
+  }
+  friend constexpr T identity_element(power_monoid<MOD, T>) noexcept {
+    return 1;
+  }
+};