feat: Add all remaining features of original Picomath PoC (#5)

Namely, a README describing the proof-of-concept,
  a custom selective loader, and some additional tests.

Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Reviewed-on: #5

test/_PocomathInstance.mjs

@@ -0,0 +1,13 @@
+import assert from 'assert'
+import PocomathInstance from '../PocomathInstance.mjs'
+
+describe('PocomathInstance', () => {
+   it('creates an instance that can define typed-functions', () => {
+      const pi = new PocomathInstance('dummy')
+      pi.install({'add': {'any,any': [[], (a,b) => a+b]}})
+      assert.strictEqual(pi.add(2,2), 4)
+      assert.strictEqual(pi.add('Kilroy', 17), 'Kilroy17')
+      assert.strictEqual(pi.add(1, undefined), NaN)
+      assert.throws(() => pi.add(1), TypeError)
+   })
+})

test/custom.mjs

@@ -3,9 +3,11 @@ import math from '../pocomath.mjs'
 import typed from 'typed-function'
 import PocomathInstance from '../PocomathInstance.mjs'
 import * as numbers from '../number/all.mjs'
+import * as numberAdd from '../number/add.mjs'
 import * as complex from '../complex/all.mjs'
 import * as complexAdd from '../complex/add.mjs'
 import * as complexNegate from '../complex/negate.mjs'
+import extendToComplex from '../complex/extendToComplex.mjs'
 
 const bw = new PocomathInstance('backwards')
 describe('A custom instance', () => {
@@ -38,4 +40,18 @@ describe('A custom instance', () => {
       assert.deepStrictEqual(
          pm.subtract({re:5, im:0}, {re:10, im:1}), {re:-5, im: -1})
    })
+
+   it("can selectively import in cute ways", async function () {
+      const cherry = new PocomathInstance('cherry')
+      cherry.install(numberAdd)
+      await extendToComplex(cherry)
+      // Now we have an instance that supports addition for number and complex
+      // and little else:
+      assert.strictEqual(cherry.add(3, 4, 2), 9)
+      assert.deepStrictEqual(
+         cherry.add(cherry.complex(3, 3), 4, cherry.complex(2, 2)),
+         math.complex(9,5))
+      assert.strictEqual('subtract' in cherry, false)
+      assert.strictEqual('negate' in cherry, false)
+   })
 })