Split off System random functions into a new Randomness module

[expenses]

This closes #1451.

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[kianenigma]

Looks quite good in general. Needs fixing a few minor issues + maybe some tests would also be good.

[bkchr]

Could you please add some sort of tests?

[expenses]

both of those would do the trick, but ideally I'd like to have something more concise.

[expenses]

Would something like an ArrayVec be useful here, given we're only storing up to 81 hashes?

[gavofyork]

Looks reasonable.

[rphmeier]

LGTM after a crate rename - it shouldn't be called randomness unless it's the only / forever guaranteed / safest way to do randomness. which it is none of

[expenses]

@rphmeier I've renamed the crate. Should documentation such as

//! # Randomness Module
//!
//! The Randomness module provides a [`random`](./struct.Module.html#method.random) function that
//! generates low-influence random values based on the block hashes from the previous 81 blocks.
//! Low-influence randomness can be useful when defending against relatively weak adversaries.

be renamed too?

[kianenigma]

@rphmeier I've renamed the crate. Should documentation such as

//! # Randomness Module
//!
//! The Randomness module provides a [`random`](./struct.Module.html#method.random) function that
//! generates low-influence random values based on the block hashes from the previous 81 blocks.
//! Low-influence randomness can be useful when defending against relatively weak adversaries.

be renamed too?

None of by business but I assume yes. Make sure to also merge master before merging this to make sure all external usages of randomness::randomness() etc. are properly refactored

[bkchr]

Some last nitpicks, otherwise looks good.

[bkchr]

Sorry, 2 last nitpicks :(

Could you please prepare a pr for polkadot using this guide: https://github.com/paritytech/substrate/blob/master/CONTRIBUTING.adoc#updating-polkadot-as-well

When the Polkadot pr is ready, I will merge this one. (Please make sure your branch in Polkadot is editable by us, so that I can finish it).