treat them like any other theano tensor)

Note: this optimization expects the state of scan to have the as_while flag
and the profile flag.

set_subtensor.

This was the cause of the 3 failing test in scan. I also revised the entire
optimization, writing a bit more comments/

variables and some other cosmetic changes.

* * *
More informative error message

ensure they don't belong to a different env.

Also update the documentation.

The scope is to make the computational graph more readable, and to move
constants in the inner graph, allowing constant folding in the inner graph
to do its job (performance).