Pretty printer for OperationAcc and PartitionedAcc

[ivogabe]

For the new pipeline, we need a pretty printer for OperationAcc and PartitionedAcc. Those two are both instantiations of PreOpenAcc from Data.Array.Accelerate.AST.Operation. Some remarks:

• The data type ranges over the type of operations op, so we need to have a type class providing the pretty printer for that type argument
• The Show instances for Acc and Exp should now print the program before desugaring, instead of the output fusion. Desugaring makes the program backend-specific, and the backend in use isn't known in the show instance. We should provide some other functionality (exposed function and/or debug flag) to print the backend-specific code.
• Alet is also used to sequence different computations. We should thus print Alet (LeftHandSideWildcard TupRunit) a b as a; b. We could also do that for non-unit wildcards in the left hand side.