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.

Libraries/LibHTML/Layout/LayoutText.cpp

@@ -20,7 +20,7 @@ LayoutText::~LayoutText()
 
 static bool is_all_whitespace(const String& string)
 {
-    for (int i = 0; i < string.length(); ++i) {
+    for (size_t i = 0; i < string.length(); ++i) {
         if (!isspace(string[i]))
             return false;
     }