Add Prelude.fieldVald

which escapes a value to be passed to Field (i.e. indent by one space,
add "." in place of empty lines).

I’m surprised this is necessary; I was expecting Field to contain the
abstract value of the field, and escape it upon printing. *shrug*
Create machine-readable copyright

based on the contents of the cabal file. Works towards #7.

 * It does not work with "--compare" (“no PackageDescription”) – David,
   can you have a look?
 * Many packages on Cabal do not have a proper copyright field, but have
   the copyright information in the LICENSE file. We should try to
   separate that out. Additionally, it would be nice to detect the
   licenses from
   https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/#license-specification
   (full texts at http://spdx.org/licenses) and not include them
   verbatim, but rather only use their shortcut.