bn: improve allocation speed for buffers

Use `allocUnsafe` to avoid deprecation warnings and improve allocation
speed.

(cherry picked from commit 9f7d21f)

lib/bn.js

@@ -535,7 +535,7 @@
 
     this.strip();
     var littleEndian = endian === 'le';
-    var res = new ArrayType(reqLength);
+    var res = allocate(ArrayType, reqLength);
 
     var b, i;
     var q = this.clone();
@@ -567,6 +567,13 @@
     return res;
   };
 
+  var allocate = function allocate (ArrayType, size) {
+    if (ArrayType.allocUnsafe) {
+      return ArrayType.allocUnsafe(size);
+    }
+    return new ArrayType(size);
+  };
+
   if (Math.clz32) {
     BN.prototype._countBits = function _countBits (w) {
       return 32 - Math.clz32(w);