buffer: directly use ArrayBuffer as the pool

Make the buffer pool an `ArrayBuffer` which is used directly,
speeding up allocation noticeably in some cases.
The only drawback happens when creating pool-based `Buffer`
instances from strings whose byte lengths got overestimated
by `Buffer.byteLength`, e.g. for base64-encoded strings
containing whitespace, where two `Buffer` instances are
being created.

This may also be useful when providing Buffer classes in
the future.

Benchmark results for `benchmark/buffers/buffer-creation.js`:

```
                                 improvement significant      p.value
len=1024 type="buffer()"             47.11 %         *** 5.202555e-12
len=1024 type="fast-alloc"           -3.41 %             3.823226e-01
len=1024 type="fast-alloc-fill"       1.11 %             7.985624e-01
len=1024 type="fast-allocUnsafe"     24.37 %         *** 4.264084e-05
len=1024 type="slow"                  4.81 %             2.634609e-01
len=1024 type="slow-allocUnsafe"      1.28 %             7.864850e-01
len=10 type="buffer()"               59.42 %         *** 9.953552e-13
len=10 type="fast-alloc"             -6.43 %             1.450524e-01
len=10 type="fast-alloc-fill"        -2.96 %             4.873766e-01
len=10 type="fast-allocUnsafe"       33.89 %         *** 6.517268e-07
len=10 type="slow"                   -1.48 %             7.357711e-01
len=10 type="slow-allocUnsafe"        0.04 %             9.939576e-01
len=2048 type="buffer()"             36.34 %         *** 3.201045e-10
len=2048 type="fast-alloc"           -4.67 %             2.172900e-01
len=2048 type="fast-alloc-fill"      -0.15 %             9.732945e-01
len=2048 type="fast-allocUnsafe"     20.13 %         *** 2.372115e-04
len=2048 type="slow"                  4.35 %             2.831340e-01
len=2048 type="slow-allocUnsafe"      1.13 %             8.055388e-01
len=4096 type="buffer()"              4.90 %             2.495340e-01
len=4096 type="fast-alloc"           -2.11 %             5.417520e-01
len=4096 type="fast-alloc-fill"      -0.29 %             9.460378e-01
len=4096 type="fast-allocUnsafe"      3.11 %             5.001959e-01
len=4096 type="slow"                  0.95 %             8.145888e-01
len=4096 type="slow-allocUnsafe"      3.74 %             4.227627e-01
len=8192 type="buffer()"              5.08 %             2.263029e-01
len=8192 type="fast-alloc"           -1.16 %             7.300235e-01
len=8192 type="fast-alloc-fill"       0.40 %             9.179919e-01
len=8192 type="fast-allocUnsafe"      5.16 %             2.591544e-01
len=8192 type="slow"                  2.57 %             5.212449e-01
len=8192 type="slow-allocUnsafe"     -3.19 %             4.699138e-01
```

PR-URL: #8302
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/buffer.js

@@ -30,17 +30,21 @@ binding.setupBufferJS(Buffer.prototype, bindingObj);
 const zeroFill = bindingObj.zeroFill || [0];
 
 function createUnsafeBuffer(size) {
+  return new FastBuffer(createUnsafeArrayBuffer(size));
+}
+
+function createUnsafeArrayBuffer(size) {
   zeroFill[0] = 0;
   try {
-    return new FastBuffer(size);
+    return new ArrayBuffer(size);
   } finally {
     zeroFill[0] = 1;
   }
 }
 
 function createPool() {
   poolSize = Buffer.poolSize;
-  allocPool = createUnsafeBuffer(poolSize);
+  allocPool = createUnsafeArrayBuffer(poolSize);
   poolOffset = 0;
 }
 createPool();
@@ -183,7 +187,7 @@ function allocate(size) {
   if (size < (Buffer.poolSize >>> 1)) {
     if (size > (poolSize - poolOffset))
       createPool();
-    var b = allocPool.slice(poolOffset, poolOffset + size);
+    var b = new FastBuffer(allocPool, poolOffset, size);
     poolOffset += size;
     alignPool();
     return b;
@@ -210,8 +214,12 @@ function fromString(string, encoding) {
 
   if (length > (poolSize - poolOffset))
     createPool();
-  var actual = allocPool.write(string, poolOffset, encoding);
-  var b = allocPool.slice(poolOffset, poolOffset + actual);
+  var b = new FastBuffer(allocPool, poolOffset, length);
+  var actual = b.write(string, encoding);
+  if (actual !== length) {
+    // byteLength() may overestimate. That’s a rare case, though.
+    b = new FastBuffer(allocPool, poolOffset, actual);
+  }
   poolOffset += actual;
   alignPool();
   return b;