Remove Lift (), Ratio a, Identity a and Const a

[phadej]

This is a fix of a mess I started. The idea is to separate
the three packages responsibilities:

• th-lift provides TemplateHaskell based deriving code for Lift
  instances (and defines Lift Name)
• th-orphans provides rest of Lift instances for
  Language.Haskell.TH namespace
• th-lift-instances provides instances for small set of core packages,
  and acts as a compat module (e.g. provides Lift ()).

Note: th-lift-instances uses TemplateHaskellQuotes only nowadays.
(Which s sometimes handy, e.g. when splicing TemplateHaskell is broken...)
That's why I removed th-lift dependency, and didn't notice the
overlapping orphan instances problem in the first place.

This patch also simplifies the build-depends as base lower-bound
is restricted by th-abstractions anyway (dead code removed from the
library too). The th-abstraction upper bound is added,
because Ryan promised to break every consumer, as type of datatypeVars
will change.

---

cc @RyanGlScott

---

If we can make this change, and then update also descriptions of th-orphans and th-lift-instances to cross-link the packages, people will know where to look for things.

I'll make a PR to add Identity, Const with (using transformers Identity when it's not in base) to th-lift-instances.

This is unfortunate situation of shuffling orphans around, but the end result should be a lot cleared. I'll (as Hackage Trustee) help to sort out diamond breakages (as th-lift and th-lift-instances don't depend on each other) if/when they occur.

[phadej]

I made the library to only use TemplateHaskellQuotes when those are enough, so th-lift-instances could depend on th-lift. Then we'll avoid downstream dependency problems.

[phadej]

ping @mboes

[phadej]

ping @mboes