Plures/xnd-ml

[Pearu Peterson] @saul, I believe that for creating Ndt pyobject we can use xnd_structinfo approach by implementing ndtype_alloc in LLVM IR. That was my initial thought and would involve getting pointer to Ndt_Type value, adding PyTypeObject struct wrapper to xnd_structinfo and implementing setter methods within xnd_structinfo. All these steps are doable.

However, we can implement the creation of a Ndt pyobject witin a C function and then expose this function to LLVM IR. It can be done either within _ndtypes.c by defining the function as extern (easy approach in short run), or (in order to keep ndtypes clean from numba related stuff) creating a dummy-python extension module that implements the required extern function (it would be just harder to get a reference to Ndt_Type). In either case the extern function is made available to LLVM IR by importing the python module and binding it as shared library to LLVM as we currently do.

If the approach works, we might need the dummy-python extension module also for other tasks that are easier to do in C (have direct access to struct members) than in LLVM IR (when pointer passing is not sufficient). Such functions can be added to xnd_structinfo, for instance, in order to keep the number of extension modules minimal.