feat: Invalidation and lazy reloading of implementations

The interface is a bit clunky, the "author" and the callback "data"
  have to be specified when adding (an) implementation(s) in order
  for this to work. But it does, and this is just a PoC, the interface
  could be cleaned up.

test/_poortf.js

@@ -21,13 +21,22 @@ describe('poortf', () => {
       assert.throws(() => add('kilroy'), TypeError)
    })
 
+   const defNumber = what => {
+      slate.addImps([args => typeof args[0] === 'number',
+                     n => 'I am not number ' + (what+n)],
+                    defNumber, what+1)
+   }
+   
    it('extends an empty tf', () => {
+      defNumber(2)
+      assert.strictEqual(slate(0, 'was here'), 'I am not number 2')
+      assert.throws(() => slate('Kilroy', 'was here'), TypeError)
       slate.addImps([
          [args => typeof args[0] === 'string', s => s + ' wuz here'],
-         [args => typeof args[0] === 'number', () => 'I am not a number']
+         [args => typeof args[0] === 'boolean', () => 'Maybe I am a number']
       ])
       assert.strictEqual(slate('Kilroy', 'was here'), 'Kilroy wuz here')
-      assert.strictEqual(slate(2, 'was here'), 'I am not a number')
+      assert.strictEqual(slate(true), 'Maybe I am a number')
       assert.throws(() => slate(['Ha!']), TypeError)
    })
 
@@ -36,6 +45,15 @@ describe('poortf', () => {
       assert.strictEqual(add('Kilroy', 23), 'Kilroy23')
       assert.throws(() => add(['Ha!'], 'gotcha'), TypeError)
    })
+   
+   it('can invalidate and lazily reload implementations', () => {
+      slate.invalidate(defNumber)
+      assert.strictEqual(slate.imps.length, 3) // reloader plus two non-number
+      assert.strictEqual(slate(1), 'I am not number 4') // 'what' is now 3!!
+      assert.strictEqual(slate('Yorlik', 7), 'Yorlik wuz here')
+      assert.strictEqual(slate(false), 'Maybe I am a number')
+      assert.strictEqual(slate.imps.length, 3) // reloader gone
+   })
 })