theano.until

(note : the implementation of make_thunk is seen by DebugMode as a python
implementation, and it sees no C implementation for that Op. Are there
checks for the C implementation that are not done for the python
implementaion ? )

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.