util: refactor format method.Performance improved.

Method format refactored to make it more maintenable, replacing the
switch by a function factory, that returns the appropiated function
given the character (d, i , f, j, s).

Also, performance when formatting an string that contains several
consecutive % symbols is improved. The test:
`const numSamples = 10000000;
const strPercents = '%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%s%%%%%%%%%%%%%%%%%i%%%%%%%%%%%%%%%%%%%%%%%%%%';
var s;
console.time('Percents');
for (let i = 0; i < numSamples; i++) {
  s = util.format(strPercents, 'test', 12);
}
console.timeEnd('Percents');`

Original time:   28399.708ms
After refactor:  23763.788ms
Improved: 16%

PR-URL: https://github.com/nodejs/node/pull/12407


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Brian White <mscdex@mscdex.net>