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

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