rand: Remove serde1 feature

[vks]

This feature does not do anything in the rand crate and was forwarded
to rand_core, where it was used for BlockRng and BlockRng64.
However, crates using this feature would use rand_core directly
instead, making the feature pointless for rand.

It is not expected to use this feature for any of the RNGs in rand.

This hopefully makes it possible to build rand on docs.rs again.
(See rust-lang/docs.rs#369.)

Now the serde1 feature does not do anything, but it is still available
for backwards compatibility.

[vks]

The failures seem to be due to "store build cache" timing out on Travis.

[dhardy]

You're right that we don't need to pass this to rand_core since e.g. rand_isaac does that when required.

I think the only reason we might want this is if we enabled serialisation for SmallRng, but currently we don't allow that anyway (with good reason since its not portable; OTOH it could still be useful for local checkpointing).

[vks]

I think the only reason we might want this is if we enabled serialisation for SmallRng, but currently we don't allow that anyway (with good reason since its not portable; OTOH it could still be useful for local checkpointing).

I don't think it makes sense to enable it for SmallRng, since we don't guarantee value stability, which also affects deserializing SmallRng. If people want to serialize the state, I think it is better to use one of the more stable RNG crates directly.