fs: add initial set of fs.promises APIs

Initial set of fs.promises APIs with documentation and one
benchmark.

PR-URL: #18297
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

Author: jasnell

benchmark/fs/bench-stat-promise.js

@@ -0,0 +1,28 @@
+'use strict';
+
+const common = require('../common');
+const fs = require('fs');
+
+const bench = common.createBenchmark(main, {
+  n: [20e4],
+  statType: ['fstat', 'lstat', 'stat']
+});
+
+async function run(n, statType) {
+  const arg = statType === 'fstat' ?
+    await fs.promises.open(__filename, 'r') : __filename;
+  let remaining = n;
+  bench.start();
+  while (remaining-- > 0)
+    await fs.promises[statType](arg);
+  bench.end(n);
+
+  if (typeof arg.close === 'function')
+    await arg.close();
+}
+
+function main(conf) {
+  const n = conf.n >>> 0;
+  const statType = conf.statType;
+  run(n, statType).catch(console.log);
+}