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/generic/all.mjs

@@ -0,0 +1 @@
+export * from './arithmetic.mjs'

src/generic/arithmetic.mjs

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+export {divide} from './divide.mjs'
+export {sign} from './sign.mjs'
+export {subtract} from './subtract.mjs'

src/generic/divide.mjs

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+export const divide = {
+   'any,any': ({multiply, invert}) => (x, y) => multiply(x, invert(y))
+}
+

src/generic/sign.mjs

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+export const sign = {
+   any: ({negate, divide, abs}) => x => {
+      if (x === negate(x)) return x // zero
+      return divide(x, abs(x))
+   }
+}