Supporting record arrays

[aldanor]

I think it would be a lfie-changing feature for some projects - basically, this would enable to export data from a Vec<Struct> almost straight to a pd.DataFrame, how cool is that?

Wonder what are the thoughts on this? Has this been considered? Otherwise, I have a bit of free time and so I could probably give it a try. This would require a separate proc-macro crate (rust-numpy-derive), and it will probably require a bit of reworking the existing dtype constructs.

(A bit of pre-history: I've implemented support for numpy record arrays in pybind11 a while ago, and then I've implemented support for custom user structs type mapping in hdf5 crate via proc macros - I think adding support for recarrays here should be somewhere inbetween the two.)

[aldanor]

(Aplogies for a dupe, just noticed #186)

[adamreichold]

@aldanor Could you close this since it is indeed a duplicate? Thanks!