Currently, Template Haskell's treatment of strictness is not enough to
cover all possible combinations of unpackedness and strictness. In
addition, it isn't equipped to deal with new features (such as
`-XStrictData`) which can change a datatype's fields' strictness during
compilation.

To address this, I replaced TH's `Strict` datatype with
`SourceUnpackedness` and `SourceStrictness` (which give the programmer a
more complete toolkit to configure a datatype field's strictness than
just `IsStrict`, `IsLazy`, and `Unpack`). I also added the ability to
reify a constructor fields' strictness post-compilation through the
`reifyConStrictness` function.

Fixes #10697.

Test Plan: ./validate

Reviewers: simonpj, goldfire, bgamari, austin

Reviewed By: goldfire, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1603

GHC Trac Issues: #10697