feat: Initial core of picomath

Implements a totally simplistic "poortf" mutable typed function and
  a picomath instance generator, as well as the very beginnings of a
  number type and one generic function and a default full picomath instance.

  Also provides some tests which serve as usage examples.

generic/subtract.js

@@ -0,0 +1,9 @@
+export default function create(pmath) {
+   const add = pmath('add')
+   const negate = pmath('negate')
+   if (!pmath.subtract) { // avoid double definition at cost of extensibility
+      pmath('subtract', [args => args.length === 2,
+         (x, y) => add(x, negate(y))])
+   }
+   return pmath.subtract
+}