[Haddock] Feature request: type signatures in index overview

[David Luposchainsky]

Hey Haddock devs,

I've recently spent a lot of time in the GHC source and was clicking
through names in the index just to find out the types for certain
identifiers. Paired with the large amount of empty space the index table
has, I thought it would be a good idea to fill that empty space with
type signatures. The index would then have three columns: name, defining
module, type. (For types/newtypes the actual definition could be filled
in instead.) What do you think?

Greetings,
David/quchen