Twig: move the loops content from loops. to the root context

Currently the content of the BEGIN blocks are stored under a loops. sub context. So if we want to use a twig for in a template we need to either not call assign_block_vars at all or iterate over loops.var_name.

Instead we should move the content to the root context (like all the other variables) and then rename the inner variable:

{% for __inner.my_loop in my_loop%}{endfor} or 

{% for my_loop.__inner in my_loop%}{endfor}

instead of 

{% for my_loop in loops.my_loop%}{endfor}