Uses Rust's div_ceil()

[brooksprumo]

Problem

Partitioned epoch rewards uses the AccountsHasher impl of div_ceil(), which we're trying to remove. Also, div_ceil() is now stable in Rust, so we should use that instead and not reimplement our own.

Summary of Changes

Use Rust's div_ceil().

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.2%. Comparing base (fa79971) to head (719ad3c).
⚠️ Report is 2833 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7096   +/-   ##
=======================================
  Coverage    83.2%    83.2%           
=======================================
  Files         854      854           
  Lines      374650   374649    -1     
=======================================
+ Hits       312046   312066   +20     
+ Misses      62604    62583   -21     

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[HaoranYi]

lgtm.