More constness
More constness

Summary:
@public

Some `YGNode*` passed as `const YGNode*`, some const refs to sub-objects introduced.
This helps selecting the desired methods in more places, i.e. `const` overloads of accessors on `YGStyle`.

Reviewed By: SidharthGuglani

Differential Revision: D15078963

fbshipit-source-id: 5013721d6edcc68f42f4504f5c331da647a294bd

janicduplessis/react-native,myntra/react-native,hammerandchisel/react-native,exponentjs/react-native,arthuralee/react-native,hammerandchisel/react-native,facebook/react-native,arthuralee/react-native,exponentjs/react-native,facebook/react-native,pandiaraj44/react-native,exponentjs/react-native,exponent/react-native,facebook/react-native,facebook/react-native,pandiaraj44/react-native,hammerandchisel/react-native,janicduplessis/react-native,exponentjs/react-native,javache/react-native,myntra/react-native,hammerandchisel/react-native,pandiaraj44/react-native,hoangpham95/react-native,pandiaraj44/react-native,hoangpham95/react-native,javache/react-native,hammerandchisel/react-native,facebook/react-native,janicduplessis/react-native,hammerandchisel/react-native,arthuralee/react-native,janicduplessis/react-native,pandiaraj44/react-native,hoangpham95/react-native,facebook/react-native,exponent/react-native,exponent/react-native,facebook/react-native,exponent/react-native,janicduplessis/react-native,myntra/react-native,hammerandchisel/react-native,myntra/react-native,javache/react-native,exponent/react-native,pandiaraj44/react-native,javache/react-native,myntra/react-native,javache/react-native,myntra/react-native,javache/react-native,hoangpham95/react-native,arthuralee/react-native,exponent/react-native,hammerandchisel/react-native,javache/react-native,facebook/react-native,myntra/react-native,hoangpham95/react-native,exponent/react-native,exponentjs/react-native,javache/react-native,myntra/react-native,javache/react-native,exponentjs/react-native,exponentjs/react-native,pandiaraj44/react-native,facebook/react-native,myntra/react-native,exponent/react-native,janicduplessis/react-native,hoangpham95/react-native,hoangpham95/react-native,exponentjs/react-native,arthuralee/react-native,hoangpham95/react-native,pandiaraj44/react-native,janicduplessis/react-native,janicduplessis/react-native