Cabal and simultaneous installations of the same package

[Edward Z. Yang]

> But if multiple versions of the same library can be linked into one
> binary, then the binary sizes will increase and therefore the memory
> usage might increase and even performance might decrease a bit if
> the number of cache misses increase.

This is not in the current proposal.  GHC knows how to deal with this,
but extending Cabal to do this is out of scope for the GSOC.

Edward