Issues with the std feature

[est31]

I have two issues with the std feature:

1. it is turned off by default. You have to turn it on manually. While optional no_std support is great, I think more people have std than have no_std, so I think it should be a default enabled feature.
2. the hasher crates don't expose the std features themselves. So there is no real way of enabling it except depending on the digest trait yourself and enabling it :/.

I'm not a fan of all of the api guidelines, but this issue is well described by them: https://github.com/rust-lang-nursery/api-guidelines/blob/5ac0bd004341f6d90f43cd7c325fe845e904c477/src/naming.md#feature-names-are-free-of-placeholder-words-c-feature

[newpavlov]

I thought about making std default feature, but decided against it, as it infects all downstream crates with default-features = false and digest/std features. I don't think that one additional string in the Cargo.toml for crate users when they need std::io functionality outweighs increased complexity for no_std crates.

@tarcieri
Your opinion?

[tarcieri]

I'm ok with an enabled-by-default std feature, so long as the API itself doesn't gain important functionality that requires std

[newpavlov]

Fixed with digest v0.8. Now std feature is enabled by default for hash crates, but still disabled by default for digest crate.