Slightly re-factor the String handling in StatTimer

This uses template strings in a couple of spots, and a buffer in the `toString` method.

src/display/display_utils.js

@@ -387,14 +387,14 @@ class StatTimer {
 
  time(name) {
  if (name in this.started) {
- warn('Timer is already running for ' + name);
+ warn(`Timer is already running for ${name}`);
  }
  this.started[name] = Date.now();
  }
 
  timeEnd(name) {
  if (!(name in this.started)) {
- warn('Timer has not been started for ' + name);
+ warn(`Timer has not been started for ${name}`);
  }
  this.times.push({
  'name': name,
@@ -407,7 +407,7 @@ class StatTimer {
 
  toString() {
  // Find the longest name for padding purposes.
- let out = '', longest = 0;
+ let outBuf = [], longest = 0;
  for (const time of this.times) {
  const name = time.name;
  if (name.length > longest) {
@@ -416,9 +416,9 @@ class StatTimer {
  }
  for (const time of this.times) {
  const duration = time.end - time.start;
- out += `${time.name.padEnd(longest)} ${duration}ms\n`;
+ outBuf.push(`${time.name.padEnd(longest)} ${duration}ms\n`);
  }
- return out;
+ return outBuf.join('');
  }
 }