Add forward declarations for individual units

[chiphogg]

Individual "au/units/*.hh" files still pull in some decently heavy
machinery. The sensible, scalable solution is to provide a
"au/units/*_fwd.hh" file corresponding to each of them. Each
"regular" units header will include its _fwd.hh header as the first
include (and we add a comment to that effect to prevent clang-format
from reordering them).

Happily, this enables us to write a somewhat meaningful test for the
forward declaraction machinery. We can now make a header file that
provides an interface with only the forward declarations. Then we call
that API inside of a test file.

Finally, this is worth updating the docs for:

• Installation: mention the fwd.hh files.
• Alternatives: mention this feature, as we're the first units
  library I know of that provides it.

All of these changes are in separate commits for the convenience of the
reviewer: I recommend starting at the first, and using n and p to
navigate them.

Fixes #232.

[geoffviola]

Makes sense. Implementation, test, and documentation changes look good.