correct array.nbytes, and add tests

[d-v-b]

AsyncArray.nbytes is very wrong, this PR corrects it (and adds tests)

closes #2575

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[rabernat]

One minor suggestion to be self consistent in our implementation.

We should reuse this function within info (where it is currently correct) rather than duplicating the same logic twice.

[dstansby]

Worth adding a note or warning that this only gives the correct answer for fixed-length types? (if I'm interpreting your TODO correctly?)

[d-v-b]

I could also just fix the implementation ... what kind of behavior would people expect when calling nbytes on an array with variable length types? maybe raise an exception?

[dstansby]

I guess an exception, or return None? Is there a precendent elsewhere in the codebase for which to do?

[d-v-b]

second question, is there any reliable way to check if a dtype is variable length, other than a big lookup table?

cc @rabernat

[d-v-b]

i'm feeling like this out of scope for this PR; numpy itself reports a (misleading) integer itemsize for variable length dtypes already, so we can lean on that bad precedent until we come up with a proper fix

[dstansby]

👍 - can you add a quick note to the docstring here though noting that it's incorrect for variable length dtypes?

[d-v-b]

have a look at the docstrings I added in 254e276