Changes needed for substrate#4612

[expenses]

See paritytech/substrate#4612. This merges in #100, but also:

• Changes rand to be dependent on the test-helpers feature, which means that the crate actually compiles with this feature turned on. This does mean that you have to run cargo test --feature test-helpers to test it though.
• Adds an Unpin impl for a few types.

[parity-cla-bot]

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[rphmeier]

Changes rand to be dependent on the test-helpers feature, which means that the crate actually compiles with this feature turned on. This does mean that you have to run cargo test --feature test-helpers to test it though.

That is pretty unergonomic. Can you explain the rationale?

[expenses]

@rphmeier I've added rand back as a dev-dependency now, so it should just work.

[expenses]

@rphmeier I need to test that.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@eb6ecd8). Click here to learn what that means.
The diff coverage is 98.05%.

@@            Coverage Diff            @@
##             master     #104   +/-   ##
=========================================
  Coverage          ?   89.09%           
=========================================
  Files             ?       10           
  Lines             ?     3155           
  Branches          ?        0           
=========================================
  Hits              ?     2811           
  Misses            ?      344           
  Partials          ?        0

Impacted Files	Coverage Δ
src/vote_graph.rs	97.33% <100%> (ø)
src/fuzz_helpers.rs	95.94% <97.48%> (ø)

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[expenses]

@rphmeier I think we're good to go here. Might be best to merge in #100 first if you're happy with that.

[rphmeier]

Solve conflicts, then merge here. Thanks!

[rphmeier]

We'll need to publish a new GRANDPA version; cc @andresilva