MAINT: Refactor DType slots into an opaque, allocated struct

Using an allocated struct (and adding some reserved empty fields)
should allow to make the size of the DType(Type) fixed, which allows
much easier definition of C-side defined DTypes (so long they are
static types).
Also organizes legacy, abstract, and parametric into flags and
introduces Macros to access all of these.

This is probably not the final word on the structure, but hiding
these into an allocated, opaque, struct seems necessary.

It also changes the allocation to use Pythons allocator rather
than malloc, just because mixing the two is not that great probably.

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