Reorganize partitioned epoch rewards runtime code

[CriesofCarrots]

Problem

Code for partitioned epoch rewards is peppered throughout bank.rs. It is very hard to trace the flows, and figure out which pieces are needed for calculation vs distribution.

Summary of Changes

Reorganize relevant code into the bank::partitioned_epoch_rewards submodule.
Reorganize unit tests into proper submodules
No functional changes, with the exception of removing a unit test that was testing a method I decided was superfluous.

Changes making things more pub were minimized. In a few cases, pub was tightened.

This is best reviewed by commit.

Smaller PRs (dare I say, partitions):

• Create submodule Reorganize partitioned epoch rewards runtime code, 1 of 5 #511
• Move EpochRewards sysvar runtime methods Move sysvar submodule; reorg partitioned epoch rewards runtime code, 2 of 5 #520
• Move distribution-related methods Move distribution methods; partitioned epoch-rewards reorg, 3 of 5 #528
• Move calculation-related methods Move calculation methods; partitioned epoch-rewards reorg, 4 of 5 #529
• Move epoch-rewards-hasher and comparison runtime methods Move remaining bits; partitioned epoch-rewards reorg, 5 of 5 #553

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.70130% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 81.8%. Comparing base (79e316e) to head (de5c43b).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #510   +/-   ##
=======================================
  Coverage    81.8%    81.8%           
=======================================
  Files         843      848    +5     
  Lines      228586   228585    -1     
=======================================
+ Hits       187124   187128    +4     
+ Misses      41462    41457    -5