Work on public API for new DType creation

[seberg]

@WarrenWeckesser the API for creating a new DType should largely involve calling this function:

numpy/numpy/core/src/multiarray/dtypemeta.c

Line 530 in 1cff159

PyArray_InitDTypeMetaFromSpec(

Currently it does not really do much, and does not work at all, but we need to work on that (and if you look at things, then input is welcome!).

numpy/numpy/core/include/numpy/ndarraytypes.h

Line 1921 in 1cff159

} dtypemeta_slots;

Is the struct defining most of the slots on the new dtypes right now. Below it are the definitions for the FromSpec function (currently private, need to be public). The struct must remain private of course.