The idea is that even when the key changes, the resulting code and compiled
module can remain the same. We avoid this by keeping a hash of the C code (as
well as of the compilation options) and when this hash matches that of an
existing module, we re-use this module instead of having two different modules
doing the same thing.

Two improvements should be made to the current implementation:
    1. Currently a module is still compiled before we detect that it is
       duplicated, while we should avoid this compilation step.
    2. It may be the case that different code or compilation options actually
       lead to the exact same compiled module. It would be nice to be able to
       detect this situation as well.

of scan to be in a certain form, which is not true anymore if constant
folding replaces them by a constant.

node in a graph.

madness of elemwise operation introduced by infershape of subtensor and by
scan and makes plots of computational graphs readable and possibly faster.

check if they still use as little memory as needed in FAST_RUN.

gradients.

the key ( in this case the right key .. for scan to figure things out).

optimizer to use a limited amount of memory instead of forcing scan to do
that. This way you can compute gradients through reduce, foldl and flodr as
well.

there is one or not.

added some type checking / an error message to scan so it will fail a little more gracefully if passed bad arguments