Self-Vote for Staking (among others.)

[kianenigma]

Treats some leftover review from #1915.

• Adds an automatic self-vote to each validator. This is an entirely artificial edge in Phragmen and will be removed at the end of the algorithm (meaning that it is NOT counted when slashing/rewarding).
• Improves the performance by removing a layer of iteration in every inner loop of the main algorithm (--at the expense of storing two sets of indexes in each Edge). This basically makes the complexity of this implementation exactly the same when compared to the reference implementation.
• Adds the Phragmen equalise() post-processing, the parameters of which are currently configured statically in the code and NOT via a storage item (I think it will be constant --should refactor to a storage item if not). Default parameters, open for suggestion: https://github.com/paritytech/substrate/pull/2078/files#diff-b5a024b4f4ae96b0bfc0681acae56709R896
• Some other minor fixes.
• Few more tests to verify the above.

Things that I am not satisfied with

• How to react to potential overflows.
• Maybe storing the direct reference of a Candidate in Edge is better (instead of storing the index-- technically same memory footprint though), given that it will probably create a mess of lifetime.

Note: This has #1951 merged into it (hence, ignore all the commits before 531ae73). Either that one will be merged before this or #1951 can be actually closed and continued here (just needs a few more paragraphs of documenting).