Previously, we were running some blocks of code at the start of every
GHCi sessions which use do-notation, something which doesn't work well
if you start GHCi with the `-XRebindableSyntax` flag on. This tweaks the
code to avoid the use of do-notation so that `-XRebindableSyntax` won't
reject it.

Test Plan: make test TEST=T13385

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #13385

Differential Revision: https://phabricator.haskell.org/D3621