Introduce Distribution.Fields module namespace

Split fields related functionality out of `Distribution.Parsec`
namespace which is not left only to handle "scalar" values.
This separation highlights that we use `parsec` for *structure* (think
JSON), and the *values* of the fields (think e.g. dates in JSON).

- New top-level `Distribution.Fields` and `Distribution.Parsec` modules
  should include most things most people would need.
- Also `Distribution.Pretty.Field` is moved to `Distribution.Fields.Pretty`,
  as now it has proper place.
- The commit is moving things around and fixing compilation errors.
- I fixed `cabal check` parse error printing as I was nearby.