Add documentation and a test for depending on zerocopy and zerocopy-derive separately

[joshlf]

We allow users to depend on zerocopy and, instead of enabling the derive feature, depend on zerocopy-derive directly. As demonstrated by this comment, doing this can have unexpected and bad ergonomics.

We should:

• add a test that makes sure the ergonomics of this use case are acceptable
• if we can't make the ergonomics as good as those of the derive feature, document what users must do in this case