feat: Switch to function-based specification of dependencies

Allows dependencies to be economically expressed and used.
  For example, see the new definition of subtract.
  Credit for the basic idea goes to James Drew, see
  https://stackoverflow.com/a/41525264

  Resolves #21.

test/_pocomath.mjs

@@ -19,7 +19,7 @@ describe('The default full pocomath instance "math"', () => {
 
    it('can be extended', () => {
       math.install({'add': {
-         '...string': [[], addends => addends.reduce((x,y) => x+y, '')]
+         '...string': () => addends => addends.reduce((x,y) => x+y, '')
       }})
       assert.strictEqual(math.add('Kilroy',' is here'), 'Kilroy is here')
    })

test/core/_PocomathInstance.mjs

@@ -4,7 +4,7 @@ import PocomathInstance from '../../src/core/PocomathInstance.mjs'
 const pi = new PocomathInstance('dummy')
 describe('PocomathInstance', () => {
    it('creates an instance that can define typed-functions', () => {
-      pi.install({add: {'any,any': [[], (a,b) => a+b]}})
+      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)

test/core/_dependencyExtractor.mjs

@@ -0,0 +1,22 @@
+import assert from 'assert'
+import dependencyExtractor from '../../src/core/dependencyExtractor.mjs'
+
+describe('dependencyExtractor', () => {
+   it('will record the keys of a destructuring function', () => {
+      const myfunc = ({a, 'b(x)': b, c: alias}) => 0
+      const params = new Set()
+      myfunc(dependencyExtractor(params))
+      assert.ok(params.has('a'))
+      assert.ok(params.has('b(x)'))
+      assert.ok(params.has('c'))
+      assert.ok(params.size === 3)
+   })
+
+   it('does not pick up anything from a regular function', () => {
+      const myfunc = arg => 0
+      const params = new Set()
+      myfunc(dependencyExtractor(params))
+      assert.ok(params.size === 0)
+   })
+
+})