feat(poortf): add concept of 'author' to an imp of a TF

And make it so that if the same author adds imps again, they replace
  the previous imps from that author.

  Use this feature to avoid the explicit check for double-inclusion in
  subtract, and test it worked.

test/_picomath.js

@@ -20,4 +20,8 @@ describe('The default full picomath instance "math"', () => {
          math.complex(11, -4))
       assert.deepStrictEqual(math.negate(math.complex(3, '8')).im, -8)
    })
+
+   it('does not double-define subtract', () => {
+      assert.deepStrictEqual(math.subtract.imps.length, 1)
+   })
 })