Improving `OsRng` access

[tarcieri]

We have several crates, some in this repo, that use traits from rand_core. An example is the aead crate:

https://docs.rs/aead/0.4.1/aead/trait.NewAead.html#method.generate_key

However, while some crates document how to use OsRng as the concrete RNG, we typically don't make this easy.

Some suggestions for improving this:

Re-export rand_core

This would make it easier to access both the traits and the built-in RNG.

It could be exported verbatim as rand_core, or perhaps we could consider making the feature name rand and re-exporting it as rand. But that might be a bit less clear than just the rand_core name, even though it's a bit uglier. (edit: so far we're just doing rand_core for clarity that it's a re-export)

Have the std feature activate rand_core/std

This would mean that rand_core is always pulled in whenever std is enabled (at least until weak feature activation ships), but would make it easy for users to access rand_core::OsRng when the std feature is enabled, e.g. as aead::rand_core::OsRng.

Implementation status

• aead: re-export rand_core #682
• cipher
• crypto-mac
• elliptic-curve
• password-hash
• signature

[tarcieri]

#682 implemented this for aead, re-exporting rand_core.

I'll go ahead and add a table for the others to the toplevel description.