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/complex/abs.mjs

@@ -0,0 +1,5 @@
+export {Types} from './Types/Complex.mjs'
+
+export const abs = {Complex: ({sqrt, add, multiply}) => z => {
+    return sqrt(add(multiply(z.re, z.re), multiply(z.im, z.im)))
+}}

src/complex/all.mjs

@@ -1,5 +1,2 @@
-export {Types} from './Types/Complex.mjs'
-export {complex} from './complex.mjs'
-export {add} from './add.mjs'
-export {negate} from './negate.mjs'
-export {subtract} from '../generic/subtract.mjs'
+export * from './native.mjs'
+export * from '../generic/arithmetic.mjs'

src/complex/native.mjs

@@ -0,0 +1,8 @@
+export {Types} from './Types/Complex.mjs'
+
+export {abs} from './abs.mjs'
+export {add} from './add.mjs'
+export {complex} from './complex.mjs'
+export {negate} from './negate.mjs'
+export {sqrt} from './sqrt.mjs'
+

src/complex/sqrt.mjs

@@ -0,0 +1,37 @@
+export { Types } from './Types/Complex.mjs'
+
+export const sqrt = {
+   Complex: ({
+      config,
+      complex,
+      multiply,
+      sign,
+      self,
+      divide,
+      add,
+      'abs(Complex)': abs,
+      subtract
+   }) => {
+      if (config.predictable) {
+         return z => {
+            const imSign = sign(z.im)
+            const reSign = sign(z.re)
+            if (imSign === 0 && reSign === 1) return complex(self(z.re))
+            return complex(
+               multiply(sign(z.im), self(divide(add(abs(z),z.re), 2))),
+               self(divide(subtract(abs(z),z.re), 2))
+            )
+         }
+      }
+      return z => {
+         const imSign = sign(z.im)
+         const reSign = sign(z.re)
+         if (imSign === 0 && reSign === 1) return self(z.re)
+         return complex(
+            multiply(sign(z.im), self(divide(add(abs(z),z.re), 2))),
+            self(divide(subtract(abs(z),z.re), 2))
+         )
+      }
+   }
+}
+