solana-program: only decode prior_voters if needed

[2501babe]

Problem

originally i wrote the VoteState parse to check is_empty and skip parsing prior_voters if it was empty. later i got rid of this as unnecessary complexity. however it turns out prior_voters may be fully half the CU cost of a typical vote account, 7-8k vs 15-16k total

Summary of Changes

add this logic back in for a 50% efficiency gain if prior_voters is empty, which is more likely than not. i wrote this to only operate with the cursor, rather than the raw input bytes, per jons suggestion

also i added bounds checking to the cursor before and after; while writing this, i discovered Cursor successfully returns 0 rather than erroring if it runs out of bounds. also i fixed a mistake with the tests where insufficient data was generated, making coverage of the full struct basically nil

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (3add40f) 81.6% compared to head (8158ed5) 81.6%.
Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #34972   +/-   ##
=======================================
  Coverage    81.6%    81.6%           
=======================================
  Files         830      830           
  Lines      224746   224767   +21     
=======================================
+ Hits       183518   183564   +46     
+ Misses      41228    41203   -25     

[joncinque]

Looks great!

Reading past the end and getting 0 is pretty wacky, but it's (shockingly) right in the implementation: https://doc.rust-lang.org/src/std/io/cursor.rs.html#320

[joncinque]

This makes a lot more sense, I probably should have said more in the last PR 😅