Deprecate and remove seqMap

[treeowl]

seqMap appears to be the only reason parallel depends on containers. It would be nice to be rid of it. The obvious replacement would be seqBifoldable. I'll have to add a Bifoldable instance for Data.Map, but that won't be a problem.

[Fuuzetsu]

is there a problem with containers dependency? it comes with ghc anyway, no?

[treeowl]

@Fuuzetsu, it does, but parallel is also a core library. When building GHC, fewer dependencies among core libraries can allow more parallelism in the build process. I don't know how much it matters in this particular case.