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/LibVT/Terminal.cpp

@@ -837,7 +837,7 @@ void Terminal::on_char(u8 ch)
 
 void Terminal::inject_string(const StringView& str)
 {
-    for (int i = 0; i < str.length(); ++i)
+    for (size_t i = 0; i < str.length(); ++i)
         on_char(str[i]);
 }