Validate input arguments

[passsy]

Prevent errors early and give useful errors for developers.
Moved argument checks inside assert blocks so they don't get executed in production

[codecov]

Codecov Report

Merging #36 into master will increase coverage by 0.68%.
The diff coverage is 70.64%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
+ Coverage   74.48%   75.16%   +0.68%     
==========================================
  Files          25       25              
  Lines        1203     1502     +299     
==========================================
+ Hits          896     1129     +233     
- Misses        307      373      +66

Impacted Files	Coverage Δ
...rc/extension/iterable_mutable_extension_mixin.dart	0% <0%> (ø)	⬆️
lib/src/extension/map_extensions_mixin.dart	100% <100%> (ø)	⬆️
lib/src/collection/set_empty.dart	80.95% <100%> (+3.17%)	⬆️
...ib/src/extension/list_mutable_extension_mixin.dart	100% <100%> (ø)	⬆️
lib/src/collection/set.dart	100% <100%> (ø)	⬆️
lib/src/collection/list.dart	100% <100%> (ø)	⬆️
...ib/src/extension/map_mutable_extensions_mixin.dart	94.11% <100%> (+3.2%)	⬆️
lib/src/collection/map_mutable.dart	85.91% <100%> (-0.66%)	⬇️
lib/src/extension/list_extension_mixin.dart	40.77% <31.25%> (-5.28%)	⬇️
lib/src/extension/iterable_extension_mixin.dart	69.81% <68.68%> (+2.26%)	⬆️
... and 6 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4c0ae8c...df0e94d. Read the comment docs.

[passsy]

I manually checked the diff. All previously covered methods have now covered argument checks. Uncovered methods have uncovered argument checks. Those added checks increase the uncovered part of the lib in this diff.