vote-program: expose handler mod under dcou

[buffalojoec]

Problem

The handler module - which contains a unifying API consisting of VoteStateHandle and VoteStateHandler - is intentionally private. However, it will be extremely useful to be able to use this unifying API both around the monorepo and in the Stake program tests.

Summary of Changes

Make the handler module public behind DCOU.

[mergify]

The Firedancer team maintains a line-for-line reimplementation of the
native programs, and until native programs are moved to BPF, those
implementations must exactly match their Agave counterparts.
If this PR represents a change to a native program implementation (not
tests), please include a reviewer from the Firedancer team. And please
keep refactors to a minimum.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.2%. Comparing base (75ee010) to head (1d867ec).
⚠️ Report is 2038 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #8312    +/-   ##
========================================
  Coverage    83.2%    83.2%            
========================================
  Files         836      836            
  Lines      367032   367032            
========================================
+ Hits       305410   305536   +126     
+ Misses      61622    61496   -126     

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[jstarry]

This is fine but there is a way to conditionally add the visibility via cfg FYI

[buffalojoec]

This is fine but there is a way to conditionally add the visibility via cfg FYI

Yes, with qualifier, but it actually throws a downstream rustc error about inline module declarations being unstable (?)

[jstarry]

ah ok, well in any case I already approved