feat: introduce Input primitive as base for FormInput (#665)

[Guria]

Closes #665

It doesn't introduce breaking changes in base package. One who used useFormInputProps hook still can continue to use it or migrate to use useInputProps instead.

It does introduce a small breaking change for reakit-system-bootstrap though. It doesn't export useFormInputProps hook anymore.

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Codecov Report

Merging #671 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #671   +/-   ##
=======================================
  Coverage   95.64%   95.64%           
=======================================
  Files         144      145    +1     
  Lines        2662     2664    +2     
  Branches      748      748           
=======================================
+ Hits         2546     2548    +2     
  Misses        116      116           

Impacted Files	Coverage Δ
packages/reakit/src/Form/FormInput.ts	100.00% <ø> (ø)
packages/reakit/src/Input/Input.ts	100.00% <100.00%> (ø)

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[diegohaz]

Thanks @Guria! :)

Usually, new components are added as experimental and targeted to the next branch. But since this is a very simple component I guess we can add it as stable. Anyway, we have to add more instructions on the contributing docs.

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[Guria]

@diegohaz have you noted that you still targeted it into next branch?

[diegohaz]

Yeah! Sorry, I didn't express myself well. We can add this component without the unstable_ prefix, but it should still be on the next branch as it's a new feature.