[Cython] gsoc: array expressions

Hey,

For my gsoc we already have some simple initial ideas, i.e.
elementwise vector expressions (a + b with a and b arrays with
arbitrary rank), I don't think these need any discussion. However,
there are a lot of things that haven't been formally discussed on the
mailing list, so here goes.

Frédéric, I am CCing you since you expressed interest on the numpy
mailing list, and I think your insights as a Theano developer can be
very helpful in this discussion.

User Interface
===========
Besides simple array expressions for dense arrays I would like a
mechanism for "custom ufuncs", although to a different extent to what
Numpy or Numba provide. There are several ways in which we could want
them, e.g. as typed functions (cdef, or external C) functions, as
lambas or Python functions in the same module, or as general objects
(e.g. functions Cython doesn't know about).
To achieve maximum efficiency it will likely be good to allow sharing
these functions in .pxd files. We have 'cdef inline' functions, but I
would prefer annotated def functions where the parameters are
specialized on demand, e.g.

 <at> elemental
def add(a, b): # elemental functions can have any number of arguments
and operate on any compatible dtype
    return a + b