`accumArray` was lazier than documented. `accum` did not have
documented strictness. The extra laziness allowed thunks to build
up in the array. Force the results of applying the accumulating
function to resolve.

Reviewers: hvr, bgamari

Reviewed By: bgamari

Subscribers: alpmestan, rwbarton, thomie, carter

GHC Trac Issues: #14785

Differential Revision: https://phabricator.haskell.org/D4403