Convert backend code to ES modules

Bundling code now with Rollup for its flat bundling and tree shaking
support. Front end code still goes through Webpack for browser support.

lib/scheduling.js

@@ -18,9 +18,9 @@
  */
 "use strict";
 
-const crypto = require("crypto");
+import crypto from "crypto";
 
-const later = require("later");
+import later from "later";
 
 function md532(str) {
   const digest = crypto
@@ -35,22 +35,22 @@ function md532(str) {
   );
 }
 
-function variance(deviceId, vrnc) {
+export function variance(deviceId, vrnc) {
   return (md532(deviceId) >>> 0) % vrnc;
 }
 
-function interval(timestamp, intrvl, offset = 0) {
+export function interval(timestamp, intrvl, offset = 0) {
   return Math.trunc((timestamp + offset) / intrvl) * intrvl - offset;
 }
 
-function parseCron(cronExp) {
+export function parseCron(cronExp) {
   const parts = cronExp.trim().split(/\s+/);
   if (parts.length === 5) parts.unshift("*");
 
   return later.schedule(later.parse.cron(parts.join(" "), true));
 }
 
-function cron(timestamp, schedule, offset = 0) {
+export function cron(timestamp, schedule, offset = 0) {
   // TODO later.js doesn't throw erorr if expression is invalid!
   const ret = [0, 0];
 
@@ -62,8 +62,3 @@ function cron(timestamp, schedule, offset = 0) {
 
   return ret;
 }
-
-exports.variance = variance;
-exports.interval = interval;
-exports.parseCron = parseCron;
-exports.cron = cron;