This augments `MetaSel` with a `Bang` field, which gives generic
programmers access to the following information about each field
selector:

* `SourceUnpackedness`: whether a field was marked `{-# NOUNPACK #-}`,
  `{-# UNPACK #-}`, or not
* `SourceStrictness`: whether a field was given a strictness (`!`) or
  laziness (`~`) annotation
* `DecidedStrictness`: what strictness GHC infers for a field during
  compilation, which may be influenced by optimization levels,
  `-XStrictData`, `-funbox-strict-fields`, etc.

Unlike in Phab:D1603, generics does not grant a programmer the ability
to "splice" in metadata, so there is no issue including
`DecidedStrictness` with `Bang` (whereas in Template Haskell, it had to
be split off).

One consequence of this is that `MetaNoSel` had to be removed, since it
became redundant. The `NoSelector` empty data type was also removed for
similar reasons.

Fixes #10716.

Test Plan: ./validate

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

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1646

GHC Trac Issues: #10716