Revive -To methods

[passsy]

Followup to #46, adding all removed functions again.

The bug in dart dart-lang/sdk#35518 still exists but it is possible to work around it. It's possible to check they incoming type at runtime. This doesn't make the API statically typed but I rather provide the methods with a runtime check to users than not providing the methods.

It will be non-breaking to change the API to be statically typed, once a bugfix landed.

Revived:

• KIterable<T>.associateWithTo
• Kiterable<T>.filterTo
• KIterable<T>.filterIndexedTo
• KIterable<T>.filterNotTo
• KIterable<T>.filterNotNullTo
• KIterable<T>.groupByTo
• KMap<T>.mapKeysTo
• KMap<T>.mapValuesTo

fix KSet equals

Also I fixed the equals implementation of Set, which wasn't working properly when comparing KSet<int> to KSet<num>

[codecov]

Codecov Report

Merging #51 into master will increase coverage by 0.55%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
+ Coverage   93.88%   94.44%   +0.55%     
==========================================
  Files          25       25              
  Lines        1538     1603      +65     
==========================================
+ Hits         1444     1514      +70     
+ Misses         94       89       -5

Impacted Files	Coverage Δ
lib/src/collection/list_mutable.dart	100% <ø> (ø)	⬆️
lib/src/extension/map_extensions_mixin.dart	100% <100%> (+3.17%)	⬆️
lib/src/collection/set.dart	100% <100%> (ø)	⬆️
lib/src/collection/set_mutable.dart	81.33% <100%> (+0.25%)	⬆️
lib/src/extension/iterable_extension_mixin.dart	96.71% <100%> (+0.62%)	⬆️

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