Added kind option to connectField.

[radekmie]

This change introduces a kind: 'leaf' | 'node' flag for the connectField, closing #717. This information is later used by AutoField, allowing it to skip one components layer (namely, Field created in connectField), effectively getting rid of one useField per field.

Rendering time (see #716) reduced noticeably, especially the upper bounds:

	v3 1f70af5	v3 895130a	v2 759a0a7
Nested min	42ms	40ms	55ms
Nested max	70ms	53ms	62ms
Flat min	51ms	43ms	22ms
Flat max	73ms	56ms	57ms

This is still a proof of concept, though. Things that are left:

• Decide if Component and options are part of the public API. We'll introduce it as an internal API, therefore it could be changed without a major version change. In the following v3.x versions it may become public (and obtain stability guarantees).
• Decide if kind is a good name and way of specifying that. Maybe something more specific like mayHaveSubfield, isLeaf or plainField would make more sense. We'll stick to kind for now.
• Mark fields with kind: 'leaf' whenever possible.
• Update other themes.
• Write tests.
• Update changelog.

[codecov]

Codecov Report

Merging #741 into v3 will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##               v3     #741   +/-   ##
=======================================
  Coverage   97.87%   97.87%           
=======================================
  Files         175      175           
  Lines        3429     3435    +6     
  Branches      625      637   +12     
=======================================
+ Hits         3356     3362    +6     
  Misses          8        8           
  Partials       65       65           

Impacted Files	Coverage Δ
packages/uniforms-antd/src/AutoField.tsx	100.00% <100.00%> (ø)
packages/uniforms-antd/src/BoolField.tsx	100.00% <100.00%> (ø)
packages/uniforms-antd/src/DateField.tsx	100.00% <100.00%> (ø)
packages/uniforms-antd/src/ErrorField.tsx	100.00% <100.00%> (ø)
packages/uniforms-antd/src/ListAddField.tsx	100.00% <100.00%> (ø)
packages/uniforms-antd/src/ListDelField.tsx	100.00% <100.00%> (ø)
packages/uniforms-antd/src/LongTextField.tsx	100.00% <100.00%> (ø)
packages/uniforms-antd/src/NumField.tsx	100.00% <100.00%> (ø)
packages/uniforms-antd/src/RadioField.tsx	90.90% <100.00%> (ø)
packages/uniforms-antd/src/SelectField.tsx	100.00% <100.00%> (ø)
... and 57 more

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