nb::ndarray: adding size, shape_ptr, stride_ptr members

[tdegeus]

Fixes #158

I was not sure how you deal with tests. I could not find tests for e.g. stride to I tried to be non-intrusive. But let me know if you want otherwise.

[wjakob]

A quick review. Two further requests:

• Please at least reference those functions somewhere from a C++ test case.
• They will still need to be mentioned in the API documentation docs/api_extra.rst

[wjakob]

The changed code looks good, but please note the request above regarding documentation + test coverage.

[wjakob]

Thanks!