This adds a data family (`URec`) and six data family instances (`UAddr`,
`UChar`, `UDouble`, `UFloat`, `UInt`, and `UWord`) which a `deriving
Generic(1)` clause will generate if it sees `Addr#`, `Char#`, `Double#`,
`Float#`, `Int#`, or `Word#`, respectively. The programmer can then
provide instances for these data family instances to provide custom
implementations for unboxed types, similar to how derived `Eq`, `Ord`,
and `Show` instances currently special-case unboxed types.

Fixes #10868.

Test Plan: ./validate

Reviewers: goldfire, dreixel, bgamari, austin, hvr, kosmikus

Reviewed By: dreixel, kosmikus

Subscribers: simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D1239

GHC Trac Issues: #10868