Proposal: deepseq should not depend on containers

[Johan Tibell]

Ticket:

http://hackage.haskell.org/trac/ghc/ticket/4868

Copy of proposal:

The  deepseq package depends on the  containers package. This forces
all packages that want to depend on deepseq in order to provide a
NFData instance for exported types, to also depend on containers.

Proposal, have containers depend on deepseq, not the other way around,
and define the NFData instances for the types in the containers
package, in the containers package.

Discussion deadline: 2 weeks.