Add visible field to Library

- for main library default is True
- for named libraries default is False
- remove Monoid Library instance; i'd rather remove emptyLibrary too
  but now these (two?) usages are explicit:
  - creating Library from BuildInfo when merging common stanzas
  - creation of dummy package in `cabal new-repl`
- Documentation is left for Francesco to write
- Field is named `visible` because we have `buildable` and `exposed`
  adjectives. `visibility: True` is unclear.