Inspired by the discussion in #14296, I've decided to
document a law which is usually in the back of my mind when I'm using
Template Haskell's `Lift` class, but isn't formally stated anywhere.
That is, every `Lift` instance should satisfy (for all `x`):

```lang=haskell
$(lift x) == x
```

Test Plan: Read it

Reviewers: austin, goldfire, bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D4050