Optics compiler warning & instance fixes

[nomisRev]

• Fix String Traversal instance
• Solve compiler warnings for Optics

[codecov]

Codecov Report

Merging #851 into master will increase coverage by 0.07%.
The diff coverage is 81.25%.

@@             Coverage Diff              @@
##             master     #851      +/-   ##
============================================
+ Coverage     44.41%   44.49%   +0.07%     
  Complexity      633      633              
============================================
  Files           294      294              
  Lines          7459     7466       +7     
  Branches        834      834              
============================================
+ Hits           3313     3322       +9     
  Misses         3844     3844              
+ Partials        302      300       -2

Impacted Files	Coverage Δ	Complexity Δ
.../arrow-optics/src/main/kotlin/arrow/optics/Fold.kt	36.58% <0%> (ø)	0 <0> (ø)	⬇️
...s/src/main/kotlin/arrow/optics/instances/string.kt	100% <100%> (+53.84%)	0 <0> (ø)	⬇️
...rc/main/kotlin/arrow/optics/instances/sequencek.kt	57.14% <83.33%> (+11.68%)	0 <0> (ø)	⬇️
...main/kotlin/arrow/optics/instances/nonemptylist.kt	66.66% <83.33%> (+4.16%)	0 <0> (ø)	⬇️
...ain/kotlin/arrow/optics/typeclasses/FilterIndex.kt	18.18% <0%> (-54.55%)	0% <0%> (ø)
...tics/src/main/kotlin/arrow/optics/instances/map.kt	94.11% <0%> (+2.94%)	0% <0%> (ø)	⬇️

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