feat(Flow): Type definition improvements

[bhough]

• Updates all modules with explicit return types.
• Fixes a variety of arity issues with types for modules with optional params.
• Remove unnecessary suppressions with latest versions of Flow.
• Moves to standard //$FlowFixMe for zero config suppressions.
• Updates typing for _statefulSelectors-based modules.
• Updates to Flow .51
• Adds /lib to ignore in .flowconfig.

Partially addresses #197

[codecov]

Codecov Report

Merging #222 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #222   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          67     67           
  Lines         370    370           
  Branches      100    100           
=====================================
  Hits          370    370

Impacted Files	Coverage Δ
src/internalHelpers/_statefulSelectors.js	100% <ø> (ø)	⬆️
src/shorthands/borderWidth.js	100% <ø> (ø)	⬆️
src/mixins/placeholder.js	100% <ø> (ø)	⬆️
src/mixins/retinaImage.js	100% <ø> (ø)	⬆️
src/color/shade.js	100% <ø> (ø)	⬆️
src/shorthands/borderColor.js	100% <ø> (ø)	⬆️
src/mixins/selection.js	100% <ø> (ø)	⬆️
src/mixins/clearFix.js	100% <ø> (ø)	⬆️
src/color/mix.js	100% <ø> (ø)	⬆️
src/shorthands/buttons.js	100% <ø> (ø)	⬆️
... and 23 more

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[codecov]

Codecov Report

Merging #222 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #222   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          68     68           
  Lines         388    386    -2     
  Branches      101    102    +1     
=====================================
- Hits          388    386    -2

Impacted Files	Coverage Δ
src/mixins/ellipsis.js	100% <ø> (ø)	⬆️
src/color/opacify.js	100% <ø> (ø)	⬆️
src/internalHelpers/_nameToHex.js	100% <ø> (ø)	⬆️
src/shorthands/animation.js	100% <ø> (ø)	⬆️
src/color/mix.js	100% <ø> (ø)	⬆️
src/internalHelpers/_hslToHex.js	100% <ø> (ø)	⬆️
src/internalHelpers/_statefulSelectors.js	100% <ø> (ø)	⬆️
src/shorthands/buttons.js	100% <ø> (ø)	⬆️
src/mixins/triangle.js	100% <ø> (ø)	⬆️
src/color/shade.js	100% <ø> (ø)	⬆️
... and 35 more

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[wtgtybhertgeghgtwtg]

 || typeof color.alpha === 'undefined'

Does this just silence the Flow error?

[bhough]

yes, though I'm still working my way through this file, as it is the sole one left keeping us from having full flow support. It is proving...troublesome.