Simd 118: extend EpochRewards sysvar

[CriesofCarrots]

Problem

The new design for partitioned epoch rewards requires more data stored in the EpochRewards sysvar.

Summary of Changes

Update the sysvar: add new fields, specify the alignment, and update clone trait

This PR does not change any rewards calculation or distribution behavior. When enable_partitioned_epoch_reward feature gate is active, the sysvar is still populated during rewards calculation and destroyed at the end of distribution.
It turns out the distribution_complete_block_height field (which I removed) wasn't being used for anything...

[joncinque]

Looks great! Just the the nit for the docs code

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.8%. Comparing base (30eecd6) to head (bab13f3).
Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master     #428     +/-   ##
=========================================
- Coverage    81.9%    81.8%   -0.1%     
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  Files         840      841      +1     
  Lines      228107   228284    +177     
=========================================
+ Hits       186851   186940     +89     
- Misses      41256    41344     +88