Cleanup shared text style lookup

#290 fixes Shared text styles lookup that was broken since switching to yoga-layout.
It does so in quite a brutal way and we believe there is a more proper solution.

See https://github.com/airbnb/react-sketchapp/pull/290#discussion_r185983026 for more context

cc @jaridmargolin

Start with PR #290 and the linked discussion for the existing Shared text styles lookup workaround and the context around the yoga-layout switch. Compare that behavior with the intended lookup approach; done means the workaround is replaced by a proper solution without breaking shared text style lookup.

Written by the indexing model from the issue text.