Implement multiload for menu_link_content

Looks like a reasonable start. I'd do a little more method name cleanup.

We also don't even need to re-write the tree in the 1st method - I think it could just be:

if ($instance) {

...

}
else {
  unset($tree[$key])
}

Here's what I have in mind - incorporating this into the branch.

:) Why it is fixed if it has not been commited.