Avoid linear lookup in unload_wkr in the Linker

I've encountered an issue with following
reproduction steps:
* `:load` a large number of modules (~2000)
* compile a BCO that depends on many other BCOs from many other modules
* `:reload`
* try to compile anything, even `1` works

Before this patch the last step takes ~5s.
It takes 80ms after.

Test Plan: harbormaster

Reviewers: simonmar, austin, hvr, bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3770