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.

test/complex/_all.mjs

@@ -0,0 +1,29 @@
+import assert from 'assert'
+import math from '../../src/pocomath.mjs'
+import PocomathInstance from '../../src/core/PocomathInstance.mjs'
+import * as complexSqrt from '../../src/complex/sqrt.mjs'
+
+describe('complex', () => {
+   it('supports sqrt', () => {
+      assert.deepStrictEqual(math.sqrt(math.complex(1,0)), 1)
+      assert.deepStrictEqual(
+         math.sqrt(math.complex(0,1)),
+         math.complex(math.sqrt(0.5), math.sqrt(0.5)))
+      math.config.predictable = true
+      assert.deepStrictEqual(math.sqrt(math.complex(1,0)), math.complex(1,0))
+      assert.deepStrictEqual(
+         math.sqrt(math.complex(0,1)),
+         math.complex(math.sqrt(0.5), math.sqrt(0.5)))
+      math.config.predictable = false
+   })
+
+   it('can bundle sqrt', async function () {
+      const ms = new PocomathInstance('Minimal Sqrt')
+      ms.install(complexSqrt)
+      await ms.importDependencies(['number', 'complex'])
+      assert.deepStrictEqual(
+         ms.sqrt(math.complex(0, -1)),
+         math.complex(ms.negate(ms.sqrt(0.5)), ms.sqrt(0.5)))
+   })
+
+})