This adds new methods `liftReadList(2)` and `liftReadListPrec(2)` to the
`Read1`/`Read2` classes which are defined in terms of `ReadPrec` instead
of `ReadS`. This also adds related combinators and changes existing
`Read1` and `Read2` instances to be defined in terms of the new methods.

Reviewers: hvr, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2379

GHC Trac Issues: #12358