buffer: align fast buffers on 8 byte boundary

Prevents alignment issues when people create a typed array from a buffer.
Unaligned loads or stores are less efficent and (on some architectures) unsafe.

lib/buffer.js

@@ -250,6 +250,7 @@ function Buffer(subject, encoding, offset) {
       this.parent = pool;
       this.offset = pool.used;
       pool.used += this.length;
+      if (pool.used & 7) pool.used = (pool.used + 8) & ~7;
     }
 
     // Treat array-ish objects as a byte array.