Add Iterable.flatMapIndexed and Iterable.flatMapIndexedTo

[passsy]

fixes #147

[codecov]

Codecov Report

Merging #178 (a78cc2e) into master (6ef4aff) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #178   +/-   ##
=======================================
  Coverage   99.07%   99.07%           
=======================================
  Files          40       40           
  Lines        1837     1843    +6     
=======================================
+ Hits         1820     1826    +6     
  Misses         17       17           

Flag	Coverage Δ
unittests	99.07% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
lib/src/collection/kt_iterable.dart	99.50% <100.00%> (+<0.01%)	⬆️

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