WTFString is one of the more costly headers to parse, and is included hundreds of times across the build. In most cases, we can simply use the Forward.h header, and only include the actual class definition in files that need to know the details of the class. This patch switches to Forward.h in header files that are currently including WTFString.h. While I removed other includes in files that I was changing WTFString.h -> Forward.h, I did not attempt to reduce includes of other items outside of those files.