Cabal package descriptions

[Ross Paterson]

I think we need an extra field Synopsis for a short summary of the
package, possibly supplemented by the longer Description.  This should
make Hackage catalogues more readable, and could also be shown by an
extended form of hc-pkg list.

(repeated) License and License-File should not be mutually exclusive.
They're both useful, for different purposes.  Perhaps License would
be more accurately called License-Type.

Perhaps Hidden-Modules should be renamed Other-Modules, now that it
is used by both libraries and executables.