Add trace injection

Authored by dfeuer on Dec 1 2017, 2:59 PM.

Description

Add trace injection

Add support for injecting runtime calls to trace in DsM. This
allows the desugarer to add compile-time information to a runtime
trace.

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: carter, thomie, rwbarton

Differential Revision: https://phabricator.haskell.org/D4162