feat: Implement signature-specifc reference

Also implements a config object that upon change, lazily invalidates
  all operations that access it.

  Also allows references to signatures with nonexistent types (which
  typed-function does not); they come back as undefined.

  Uses these features to implement sqrt for number and complex.

  Resolves #7.

src/number/abs.mjs

@@ -0,0 +1,3 @@
+export {Types} from './Types/number.mjs'
+
+export const abs = {number: () => n => Math.abs(n)}