Allow serde to be turned off without turning off std

[est31]

Fixes #59

[pczarn]

With this change, such code in a downstream Cargo.toml:

bit-vec = { version = "0.6", features = ["serde_std"] }

is exactly equivalent to

bit-vec = { version = "0.6", features = ["serde"] }
serde = "1.0"

Passing a 'serde' feature makes sense only if you intend to depend on it. However, we cannot be sure downstream crates are reasonable.

What's more, serde has the alloc feature, which we must pass, because we use the alloc crate on no_std.

One decent option is to have this

serde_std = ["std", "serde/std"]
serde_no_std = ["serde/alloc"]

Another, more transparent option would be a build script to configure it for us. Unfortunately, this is impossible, build scripts cannot deeply interact with cargo: rust-lang/cargo#5499.

[pczarn]

Yet another option is to open an issue for Cargo about the semantics of enabling a dependency's feature. We may have an option to say serde/std if serde.