aead: do not enable alloc by default?

[newpavlov]

Right now only aead enables alloc by default, which forces implementation crates to use default-features = false. Removing a default feature is a breaking change, so it would have to be done in v0.4.

[tarcieri]

This seems like an artifact of when alloc was mandatory. I'd agree with disabling it per default.

(as it were, signature enables std by default as well, although fixing this is tracked in #237)

[tarcieri]

One important change we'd have to do along with this is very clearly document enabling the alloc feature to use the allocating APIs, which are prominently featured in each crate.