niteria authored Jun 27, 2017 and Ben Gamari committed Jun 27, 2017

When loading/reloading with a large number of modules
(>5000) the cost of linear lookups becomes significant.

The changes here made `:reload` go from 6s to 1s on my
test case.

The bottlenecks were `needsLinker` in `DriverPipeline` and
`getModLoop` in `GhcMake`.

Test Plan: ./validate

Reviewers: simonmar, austin, bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3646