feat: Invalidation and lazy reloading of implementations

The interface is a bit clunky, the "author" and the callback "data"
  have to be specified when adding (an) implementation(s) in order
  for this to work. But it does, and this is just a PoC, the interface
  could be cleaned up.

generic/subtract.js

@@ -4,6 +4,6 @@ export default function create(pmath) {
    return pmath(
       'subtract',
       [args => args.length === 2, (x, y) => add(x, negate(y))],
-      create)
+      create, pmath)
    return pmath.subtract
 }