base: Explicitly mark Data.Either.{left,right} as INLINABLE

Test Plan: read it

Reviewers: dfeuer, austin, hvr, nomeata

Reviewed By: dfeuer, nomeata

Subscribers: nomeata, rwbarton, thomie

GHC Trac Issues: #13689

Differential Revision: https://phabricator.haskell.org/D3576