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)
+   })
+})