Fix signature of mapNotNull and friends

[passsy]

Fixes #192

Move filterNotNullTo, mapIndexedNotNull, mapIndexedNotNullTo, mapNotNull, mapNotNullTo from
extension RequireNoNullsKtIterableExtension<T> on KtIterable<T?> to
extension KtIterableExtensions<T> on KtIterable<T>

Meaning those now work well on non-nullable types.

Only upcasts from T? to T (nullable to non-nullable) stay in RequireNoNullsKtIterableExtension

[codecov]

Codecov Report

Merging #197 (529665d) into master (9d2abc0) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #197      +/-   ##
==========================================
+ Coverage   99.22%   99.44%   +0.22%     
==========================================
  Files          40       40              
  Lines        1931     2695     +764     
==========================================
+ Hits         1916     2680     +764     
  Misses         15       15              

Flag	Coverage Δ
unittests	99.44% <100.00%> (+0.22%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
lib/src/collection/kt_iterable.dart	99.63% <100.00%> (+0.08%)	⬆️

... and 6 files with indirect coverage changes

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