Summary:
Serialization of BCOs is slow, but we can parallelise it when using
ghci -j<n>.  It parallelises nicely, saving multiple seconds off the
link time in a large example I have.

Test Plan:
* validate
* `ghci -fexternal-interpreter` in `nofib/real/anna`

Reviewers: niteria, bgamari, ezyang, austin, hvr, erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1877

GHC Trac Issues: #11100