Introduce FIeldGrammar.

This commit reworks how GenericPackageDescription is parsed from
`[Field Position]` and pretty-printed to `Doc`.

This also fixes few issues:
- Fix #4697: `cabal format` doesn't output custom-setup stanza (nor
  foreign-lib stanzas)
- Fix #4719: `parse . pretty . parse = parse`  for all Hackage cabal files.
    - `parser-hackage-tests roundtrip` is the test program.

The handling of `license-file` and `license-files` is changed.
Now they behave the same.