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/sqrt.mjs

@@ -0,0 +1,14 @@
+export { Types } from './Types/number.mjs'
+
+export const sqrt = {
+   number: ({config, complex, 'self(Complex)': complexSqrt}) => {
+      if (config.predictable || !complexSqrt) {
+         return n => isNaN(n) ? NaN : Math.sqrt(n)
+      }
+      return n => {
+         if (isNaN(n)) return NaN
+         if (n >= 0) return Math.sqrt(n)
+         return complexSqrt(complex(n))
+      }
+   }
+}