feat: Implement subtypes

This should eventually be moved into typed-function itself, but for
  now it can be implemented on top of the existing typed-function.

  Uses subtypes to define (and error-check) gcd and lcm, which are only
  defined for integer arguments.

  Resolves #36.

src/bigint/quotient.mjs

@@ -0,0 +1,13 @@
+export * from './Types/bigint.mjs'
+
+/* Returns the best integer approximation to n/d */
+export const quotient = {
+   'bigint,bigint': ({'sign(bigint)': sgn}) => (n, d) => {
+      const dSgn = sgn(d)
+      if (dSgn === 0n) return 0n
+      if (sgn(n) === dSgn) return n/d
+      const quot = n/d
+      if (quot * d == n) return quot
+      return quot - 1n
+   }
+}