Fix stack overflow for when calling KList.single

[passsy]

Adds faster routes for KLists.single() without allocating a KIterator

[codecov]

Codecov Report

Merging #44 into master will increase coverage by 1.44%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
+ Coverage    92.6%   94.05%   +1.44%     
==========================================
  Files          25       25              
  Lines        1502     1515      +13     
==========================================
+ Hits         1391     1425      +34     
+ Misses        111       90      -21

Impacted Files	Coverage Δ
lib/src/extension/iterable_extension_mixin.dart	96.22% <100%> (+3.13%)	⬆️
lib/src/extension/list_extension_mixin.dart	99.15% <100%> (+0.13%)	⬆️

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