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/number/_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 numberSqrt from '../../src/number/sqrt.mjs'
+import * as complex from '../../src/complex/all.mjs'
+import * as numbers from '../../src/number/all.mjs'
+describe('number', () => {
+   it('supports sqrt', () => {
+      assert.strictEqual(math.sqrt(4), 2)
+      assert.strictEqual(math.sqrt(NaN), NaN)
+      assert.strictEqual(math.sqrt(2.25), 1.5)
+      assert.deepStrictEqual(math.sqrt(-9), math.complex(0, 3))
+      math.config.predictable = true
+      assert.strictEqual(math.sqrt(-9), NaN)
+      math.config.predictable = false
+      assert.deepStrictEqual(math.sqrt(-0.25), math.complex(0, 0.5))
+   })
+
+   it('supports sqrt by itself', () => {
+      const no = new PocomathInstance('Numbers Only')
+      no.install(numberSqrt)
+      assert.strictEqual(no.sqrt(2.56), 1.6)
+      assert.strictEqual(no.sqrt(-17), NaN)
+      no.install(complex)
+      no.install(numbers)
+      assert.deepStrictEqual(no.sqrt(-16), no.complex(0,4))
+   })
+
+})