Replace calls to `ptext . sLit` with `text`

Authored by jstolarek.

Description

Replace calls to ptext . sLit with text

Summary:
In the past the canonical way for constructing an SDoc string literal was the
composition ptext . sLit. But for some time now we have function text that
does the same. Plus it has some rules that optimize its runtime behaviour.
This patch takes all uses of ptext . sLit in the compiler and replaces them
with calls to text. The main benefits of this patch are clener (shorter) code
and less dependencies between module, because many modules now do not need to
import FastString. I don't expect any performance benefits - we mostly use
SDocs to report errors and it seems there is little to be gained here.

Test Plan: ./validate

Reviewers: bgamari, austin, goldfire, hvr, alanz

Subscribers: goldfire, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D1784