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/man.cpp

@@ -57,7 +57,7 @@ int main(int argc, char* argv[])
     }
     dbg() << "Loading man page from " << file->filename();
     auto buffer = file->read_all();
-    String source { (char*)buffer.data(), buffer.size() };
+    String source { (const char*)buffer.data(), (size_t)buffer.size() };
 
     printf("%s(%s)\t\tSerenity manual\n", name.characters(), section.characters());