Expose a few ArrayDescriptor properties and option for mutable flags

[fwilliams]

Description

This change exposes a few elements in the PyArrayDescr struct which I am using in another project.

It also exposes a mutable version of flags() in pybind11::array(), which is useful when you need to steal the memory from a numpy array accross python calls.

[Skylion007]

Huh, why is this typedef necessary?

[Skylion007]

no need for iostream include

[fwilliams]

whoops, had some old code remaining from my fork that I forgot to remove. Fixed this.

[rwgk]

@fwilliams could you please add tests for the new methods?

Especially the mutable flags method: I'm not very familiar with numpy, do we have someone with more background than I have? @henryiii, @ax3l?

I had a quick look here:

https://numpy.org/doc/stable/reference/generated/numpy.ndarray.flags.html

What stands out to me:

or by calling ndarray.setflags
...
The array flags cannot be set arbitrarily:

My gut feeling is that it would be safer not to expose the direct mutable reference, but to add setflags support.

which is useful when you need to steal the memory from a numpy array accross python calls

That sounds very dangerous (bug prone) and would need careful explanation and unit testing.

[Skylion007]

I agree with @rwgk about it's probably better to use set_flags method. @rwgk we may want to update our code to also set the flags this way when possible.