as_buffer() support for Object values.

[TkTech]

We now support as_buffer() #59 which drastically improves performance when loading arrays from JSON into numpy.

We should also support as_buffer() for Objects, which would retrieve the values as an array. This is to support calling it like this:

https://github.com/riddell-stan/pystan-next/blob/1e027a8bded88d51c6b957a841b859938c0ac86d/stan/fit.py#L93

For @riddell-stan's use case, this should use less memory and be roughly 4x faster (since we also void the iterator creation for values())

[TkTech]

Additionally, we should make sure this works for round-tripping into a Panda's dataframe.

[riddell-stan]

Fixing this would be wonderful. Glad it's on your radar.

FYI: we renamed the repository. Here's an updated link: https://github.com/stan-dev/pystan/blob/1ffd41a5827c4d54c31205e350f0e698bcb2d87f/stan/fit.py#L93

[fabiopicchi]

Do you need help with this one?