Improved performance of PrimitiveGrowable for nulls (-10%)

[jorgecarleitao]

• Added docs
• standardized design on extending nulls (-10% for primitive arrays)

[codecov]

Codecov Report

Merging #434 (3df71cd) into main (24f6194) will increase coverage by 0.05%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main     #434      +/-   ##
==========================================
+ Coverage   80.78%   80.84%   +0.05%     
==========================================
  Files         372      372              
  Lines       22645    22729      +84     
==========================================
+ Hits        18294    18375      +81     
- Misses       4351     4354       +3     

Impacted Files	Coverage Δ
src/array/growable/binary.rs	94.73% <ø> (ø)
src/array/growable/boolean.rs	81.48% <ø> (ø)
src/array/growable/fixed_binary.rs	68.75% <ø> (ø)
src/array/growable/list.rs	96.61% <ø> (ø)
src/array/growable/mod.rs	48.00% <ø> (ø)
src/array/growable/null.rs	53.84% <ø> (ø)
src/array/growable/structure.rs	88.23% <ø> (ø)
src/array/growable/utf8.rs	100.00% <ø> (ø)
src/array/growable/utils.rs	100.00% <ø> (ø)
src/array/growable/dictionary.rs	69.56% <75.00%> (+2.89%)	⬆️
... and 6 more

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