netCDF4: support for structured arrays as attribute values; serialize as "compound types"

Code Sample, a copy-pastable example if possible

A "Minimal, Complete and Verifiable Example" will make it much easier for maintainers to help you:
http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports

ds.attrs = dict(a=dict(b=2))
ds.to_netcdf(outfile)

...

~/miniconda3/envs/ana/lib/python3.6/site-packages/xarray/backends/api.py in check_attr(name, value)
    158                             'a string, an ndarray or a list/tuple of '
    159                             'numbers/strings for serialization to netCDF '
--> 160                             'files'.format(value))
    161 
    162     # Check attrs on the dataset itself

TypeError: Invalid value for attr: {'b': 2} must be a number, a string, an ndarray or a list/tuple of numbers/strings for serialization to netCDF files

Problem description

I'm not entirely sure if this should be possible, but it seems like it should be from this email: https://www.unidata.ucar.edu/support/help/MailArchives/netcdf/msg10502.html

Nested attributes would be nice as a way to namespace metadata.

Expected Output

Netcdf with nested global attributes.

Output of xr.show_versions()

The traceback identifies xarray/backends/api.py and check_attr; start there, then trace the netCDF4 serialization path for structured arrays and compound types. Done means the supplied nested-attribute example writes a netCDF file with nested global attributes, with the behavior verified.