serde: enable no-std support

[nim65s]

Hi,

This PR enable no-std support on serde, ref. https://serde.rs/no-std.html

Otherwise, trying to compile a project with bitflags 2.0.0-rc.1 -F serde on --target=thumbv7m-none-eabi currently hangs with a huge number of eg.

error[E0432]: unresolved imports `self::core::cmp`, `self::core::iter`, `self::core::mem`, `self::core::num`, `self::core::ptr`, `self::core::slice`, `self::core::str`
   --> /home/nim/.cargo/registry/src/github.com-1ecc6299db9ec823/serde-1.0.152/src/lib.rs:167:26

[KodrAus]

Thanks @nim65s! I don't think we need the derive feature here since we only derive serde traits in tests.

[KodrAus]

Thanks @nim65s! This is a great catch 👍