Feature derive and bits are not exclusive

[Mirko-von-Leipzig]

Using the following does not compile:

ff = { version = "0.11.0", default-features = false, features = ["derive", "alloc"] }

because derive depends on bitvec which is only included with the bits feature enabled.

Compile error, and relevant code snippet:

<...>.cargo/registry/src/github.com-1ecc6299db9ec823/ff-0.11.0/src/lib.rs:269:14
    |
269 |     pub use {bitvec, byteorder, rand_core, subtle};
    |              ^^^^^^ no external crate `bitvec`

ff-0.11.0/src/lib.rs:

/// Functions and re-exported crates used by the [`PrimeField`] derive macro.
#[cfg(feature = "derive")]
#[cfg_attr(docsrs, doc(cfg(feature = "derive")))]
pub mod derive {
    pub use crate::arith_impl::*;
    pub use {bitvec, byteorder, rand_core, subtle};
}

[Mirko-von-Leipzig]

On further investigation, it appears ff_derive actually depends on bitvec without listing it as a dependency.

[str4d]

Fixing this properly blocks on #78, due to the need for weak dependency features.