Add faster elementAtOrNull implementation for List

[passsy]

While Iterable doesn't have necessarily a length (might be infinite), a list has a fixed size and a faster lookup is possible

Fixes #124

[codecov]

Codecov Report

Merging #126 (439f24e) into master (c935525) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   96.17%   96.19%   +0.02%     
==========================================
  Files          10       10              
  Lines         653      657       +4     
==========================================
+ Hits          628      632       +4     
  Misses         25       25              

Impacted Files	Coverage Δ
lib/src/list.dart	82.22% <100.00%> (+1.73%)	⬆️

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