Needed for the GHC build system

It now works with Extensions rather than KnownExtensions

The knownExtensions list had already got out of sync with the datatype.
Now we use an Enum instance to get the list of constructors.
Also, having the actual extensions in a separate type than UnknownExtension
and EnableExtension makes it easier to add extension disabling support
later.

Should live in the ghc testsuite but no harm having it here too.

Add Default-Language field, and change Extensions to Default-Extensions.

Something in the Windows build was complaining about the lack of a
final newline.

Make it clear it is a program we are talking about, not a library.

Incidentally, should fixe ticket #43.

New fields default-language and other-languages for specifying the
base languages for the package, e.g. Haskell98, Haskell2010
New fields default-extensions and other-extensions for the language
extensions. Separate from the old extensions field.
The separation lets us express the difference between declaring to
the outside world that a package uses certain languages or extensions
and whether certain languages or extensions should be applied to
all modules in the package component.