fix: Separate typed instance for each PocomathInstance (#15)

Also starts each PocomathInstance with no types at all, and uses the new
  situation to eliminate the need for a Complex "base case".

  Resolves #14.
  Resolves #13.

Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Reviewed-on: #15

src/generic/subtract.mjs

@@ -0,0 +1,3 @@
+export const subtract = {
+   'any,any': [['add', 'negate'], ref => (x,y) => ref.add(x, ref.negate(y))]
+}