Smarter field state management during insert and remove

[erikras]

This should fix a whole slew of bugs mentioned in final-form/react-final-form-arrays#66.

[codecov]

Codecov Report

Merging #29 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #29   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          11     11           
  Lines         101    120   +19     
  Branches       19     22    +3     
=====================================
+ Hits          101    120   +19

Impacted Files	Coverage Δ
src/shift.js	100% <100%> (ø)	⬆️
src/unshift.js	100% <100%> (ø)	⬆️
src/insert.js	100% <100%> (ø)	⬆️
src/remove.js	100% <100%> (ø)	⬆️

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