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/gpn-heuristic.js

@@ -28,7 +28,7 @@ const UNDISCOVERED_DEPTH = 7;
 // flags is an int where its bits mark the segments in the pattern that
 // need refreshing. Leading 0s indicate that the pattern up to that
 // point has been discovered.
-function estimateGpnCount(gpnPatterns, depth = 0) {
+export function estimateGpnCount(gpnPatterns, depth = 0) {
   const children = {};
   const wildcardChildren = [];
   let wildcardDiscovered = false;
@@ -77,5 +77,3 @@ function estimateGpnCount(gpnPatterns, depth = 0) {
 
   return gpnCount;
 }
-
-exports.estimateGpnCount = estimateGpnCount;