benchmark: add benchmarks for `Buffer.from()`

Adds benchmarks for `Buffer.from()` and its various
argument combinations.

Ref: https://github.com/nodejs/node/issues/8733
PR-URL: https://github.com/nodejs/node/pull/8738
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

benchmark/buffers/buffer-from.js

@@ -0,0 +1,86 @@
+'use strict';
+
+const common = require('../common.js');
+const assert = require('assert');
+const bench = common.createBenchmark(main, {
+  source: [
+    'array',
+    'arraybuffer',
+    'arraybuffer-middle',
+    'buffer',
+    'uint8array',
+    'string',
+    'string-base64'
+  ],
+  len: [10, 2048],
+  n: [1024]
+});
+
+function main(conf) {
+  const len = +conf.len;
+  const n = +conf.n;
+
+  const array = new Array(len).fill(42);
+  const arrayBuf = new ArrayBuffer(len);
+  const str = 'a'.repeat(len);
+  const buffer = Buffer.allocUnsafe(len);
+  const uint8array = new Uint8Array(len);
+
+  var i;
+
+  switch (conf.source) {
+    case 'array':
+      bench.start();
+      for (i = 0; i < n * 1024; i++) {
+        Buffer.from(array);
+      }
+      bench.end(n);
+      break;
+    case 'arraybuffer':
+      bench.start();
+      for (i = 0; i < n * 1024; i++) {
+        Buffer.from(arrayBuf);
+      }
+      bench.end(n);
+      break;
+    case 'arraybuffer-middle':
+      const offset = ~~(len / 4);
+      const length = ~~(len / 2);
+      bench.start();
+      for (i = 0; i < n * 1024; i++) {
+        Buffer.from(arrayBuf, offset, length);
+      }
+      bench.end(n);
+      break;
+    case 'buffer':
+      bench.start();
+      for (i = 0; i < n * 1024; i++) {
+        Buffer.from(buffer);
+      }
+      bench.end(n);
+      break;
+    case 'uint8array':
+      bench.start();
+      for (i = 0; i < n * 1024; i++) {
+        Buffer.from(uint8array);
+      }
+      bench.end(n);
+      break;
+    case 'string':
+      bench.start();
+      for (i = 0; i < n * 1024; i++) {
+        Buffer.from(str);
+      }
+      bench.end(n);
+      break;
+    case 'string-base64':
+      bench.start();
+      for (i = 0; i < n * 1024; i++) {
+        Buffer.from(str, 'base64');
+      }
+      bench.end(n);
+      break;
+    default:
+      assert.fail(null, null, 'Should not get here');
+  }
+}