Removed un-necessary allocation in assign_ops

[jorgecarleitao]

This PR removes unnecessary allocation on assign_ops.

When converting PrimitiveArray -> MutablePrimitiveArray, we need to check that both the values and validities are unique refs. However, when operating over values and validities independently, we do not need a mutable ref to both at the same time.

A common case where this was hitting was when rhs had a validity and lhs did not. In this case, we first cloned the rhs's validity for lhs, but them tried to get a mut ref to both the validity and the values (to only use the values). This would naturally fail, causing us to have to re-use the lhs.

This PR adds PrimitiveArray::get_mut_values and removes (un-released) functions from PrimitiveArray, since the function get_mut_values is more generic.

[codecov]

Codecov Report

Merging #1085 (75c7bfb) into main (16d01ef) will decrease coverage by 0.05%.
The diff coverage is 61.90%.

@@            Coverage Diff             @@
##             main    #1085      +/-   ##
==========================================
- Coverage   81.14%   81.08%   -0.06%     
==========================================
  Files         367      367              
  Lines       35316    35281      -35     
==========================================
- Hits        28657    28609      -48     
- Misses       6659     6672      +13     

Impacted Files	Coverage Δ
src/array/boolean/mod.rs	80.86% <ø> (-4.01%)	⬇️
src/array/primitive/mutable.rs	95.57% <ø> (+0.63%)	⬆️
src/compute/arity_assign.rs	64.00% <46.42%> (-10.08%)	⬇️
src/buffer/immutable.rs	83.95% <87.50%> (-4.94%)	⬇️
src/array/primitive/mod.rs	79.53% <100.00%> (-2.13%)	⬇️
src/bitmap/assign_ops.rs	83.06% <0.00%> (-3.23%)	⬇️
... and 5 more

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[ritchie46]

However, when operating over values and validities independently, we do not need a mutable ref to both at the same time.

Good observation.