Many basic functions in `GHC.Arr` were unreasonably lazy about
performing array lookups. This could lead to useless thunks
at best and memory leaks at worst. Use eager lookups where
they're obviously appropriate.

Reviewers: bgamari, hvr

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4773