Cabal package descriptions

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.