Export pure isNothing() function

[screendriver]

import { isNothing, nothing, Maybe } from 'true-myth/maybe';

const myJust: Maybe<string> = nothing();

if (isNothing(myJust)) {
  console.log('No myJust.value available');
}

This will also "fix" #305 (comment) somehow. isNothing will then show up in https://true-myth.js.org/modules/maybe.html

[chriskrycho]

Mind rebasing now that #331 has landed? Thanks!

[screendriver]

Rebased 🙂