perf_hooks: implementation of the perf timing API

An initial implementation of the Performance Timing API for Node.js.
This is the same Performance Timing API implemented by modern browsers
with a number of Node.js specific properties. The User Timing mark()
and measure() APIs are implemented, garbage collection timing, and
node startup milestone timing.

```js
const { performance } = require('perf_hooks');

performance.mark('A');
setTimeout(() => {
  performance.mark('B');
  performance.measure('A to B', 'A', 'B');
  const entry = performance.getEntriesByName('A to B', 'measure')[0];
  console.log(entry.duration);
}, 10000);
```

The implementation is at the native layer and makes use of uv_hrtime().
This should enable *eventual* integration with things like Tracing
and Inspection.

The implementation is extensible and should allow us to add new
performance entry types as we go (e.g. for measuring i/o perf,
etc).

Documentation and a test are provided.

PR-URL: #14680
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

doc/api/_toc.md

@@ -32,6 +32,7 @@
 * [Net](net.html)
 * [OS](os.html)
 * [Path](path.html)
+* [Performance Hooks](perf_hooks.html)
 * [Process](process.html)
 * [Punycode](punycode.html)
 * [Query Strings](querystring.html)

doc/api/all.md

@@ -27,6 +27,7 @@
 @include net
 @include os
 @include path
+@include perf_hooks
 @include process
 @include punycode
 @include querystring