Simplify some things related to boolean arrays

[andyleiserson]

• Add a BooleanArray trait as shorthand for SharedValue + CustomArray<Element = Boolean> (and a few other traits that are always implemented for boolean arrays).
• Delete ArrayAccessRef (not needed after #1049 and perhaps impl DerefMut for BitDecomposed).
• Remove ArrayAccess for Galois fields (perhaps not needed after fix for #1079).
• Remove some generics from some AdditiveShare-related things that are only used with Boolean arrays.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.20%. Comparing base (afcbcc5) to head (94d83db).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1115 +/- ## ========================================== - Coverage 91.21% 91.20% -0.01% ========================================== Files 187 187 Lines 26480 26422 -58 ========================================== - Hits 24153 24098 -55 + Misses 2327 2324 -3 ```

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