The new instance resolves to `s ~ [Char]` as soon as we know that `s ~
[a]`, to avoid certain functions (like (++)) causing a situation where
`a` is ambiguous and (currently) unable to be defaulted.

Reviewers: #core_libraries_committee, hvr, austin, bgamari

Reviewed By: hvr, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1572

GHC Trac Issues: #10814