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.

Userland/md.cpp

@@ -30,7 +30,7 @@ int main(int argc, char* argv[])
     auto buffer = file->read_all();
     dbg() << "Read size " << buffer.size();
 
-    String input { (char*)buffer.data(), buffer.size() };
+    String input { (const char*)buffer.data(), (size_t)buffer.size() };
     MDDocument document;
     success = document.parse(input);