Fix compile errors in examples and docs in valuable crate

[alexanderkjall]

How to reproduce the errors:

$ git clone https://github.com/tokio-rs/valuable.git
Cloning into 'valuable'...
remote: Enumerating objects: 1365, done.
remote: Counting objects: 100% (406/406), done.
remote: Compressing objects: 100% (145/145), done.
remote: Total 1365 (delta 339), reused 262 (delta 259), pack-reused 959
Receiving objects: 100% (1365/1365), 295.04 KiB | 715.00 KiB/s, done.
Resolving deltas: 100% (834/834), done.
$ cd valuable/valuable
$ cargo test

Example error:

error: cannot find derive macro `Valuable` in this scope
 --> valuable/examples/derive.rs:7:10
  |
7 | #[derive(Valuable)]
  |          ^^^^^^^^
  |
note: `Valuable` is imported here, but it is only a trait, without a derive macro
 --> valuable/examples/derive.rs:1:5
  |
1 | use valuable::Valuable;
  |     ^^^^^^^^^^^^^^^^^^

Those are due to that the feature valuable-derive might not have been enabled, and are fixed by adding a #[cfg(not(feature = "valuable-derive",))] around them.

[taiki-e]

Thanks for the PR!

[taiki-e]

It is fine that documentation or tests require all features. See rust-lang/futures-rs#2216 for more.

[alexanderkjall]

The background of this patch was that the Debian build system does build crates with the different features both enabled and not enabled, so in order to avoid errors there i wrote this patch: https://salsa.debian.org/rust-team/debcargo-conf/-/blob/master/src/valuable/debian/patches/fix-example-and-doc-compile-errors.patch

And I thought that I should also provide it upstream to be a good community member :)

[alexanderkjall]

to follow up, does this mean that you are not interested in this PR? That isn't a big problem for me, we can just close it if it won't get merged.

[alexanderkjall]

Realized that this was neither a complete fix nor a good enough one, closing it for now.