Ben Gamari authored Jul 24, 2017 and Ben Gamari committed Jul 24, 2017

While debugging #14005 I noticed that unpackCStringUtf8# was allocating
a thunk for each Unicode character that it unpacked. This seems hardly
worthwhile given that the thunk's closure will be at least three words,
whereas the Char itself will be only two and requires only a bit of bit
twiddling to construct.

Test Plan: Validate

Reviewers: simonmar, austin

Subscribers: dfeuer, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3769