An InlineNative for String.isEmpty, so it's not slower than length() == 0.

An InlineNative for String.isEmpty, so it's not slower than length() == 0.

Before:

       benchmark  ns logarithmic runtime
         IsEmpty  115 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
LengthEqualsZero   21 XXXXX||||||||||||||

With C intrinsic:

          IsEmpty  30 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
 LengthEqualsZero  20 XXXXXXXXXXXXXXXXXXXX||||||

With assembler intrinsic:

          IsEmpty  15 XXXXXXXXXXXXXXXXXXXX||||||
 LengthEqualsZero  21 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

(All times on passion.)

Change-Id: Ifcc37fe7b8efdd377675a448e0085e490d6767bc