Added (more) missing instances for Identity and Const

  * `Identity` and `Const` now have `Num`, `Real`, `Integral`,
    `Fractional`, `Floating`, `RealFrac` and `RealFloat` instances

  * `Identity` and `Const` now have `Bits` and `FiniteBits` instances

  * `Identity` and `Const` now have `IsString` instances

Reviewers: RyanGlScott, austin, hvr, bgamari, ekmett

Reviewed By: ekmett

Subscribers: nomeata, ekmett, RyanGlScott, thomie

Differential Revision: https://phabricator.haskell.org/D2079

GHC Trac Issues: #11790

(cherry picked from commit 8b57cac5)