AK: Use size_t for the length of strings

Using int was a mistake. This patch changes String, StringImpl,
StringView and StringBuilder to use size_t instead of int for lengths.
Obviously a lot of code needs to change as a result of this.

AK/URL.cpp

@@ -31,7 +31,7 @@ bool URL::parse(const StringView& string)
     Vector<char, 256> buffer;
     State state { State::InProtocol };
 
-    int index = 0;
+    size_t index = 0;
 
     auto peek = [&] {
         if (index >= string.length())