seq and pseq unsafety and ambiguities

[jmcarthur]

base's seq and pseq functions break parametricity, which, among other things, is required for some optimizations like deforestation to work. Therefore, I propose that they be considered unsafe functions and that we adopt the unsafe naming conventions for them. Furthermore, since seq doesn't actually guarantee in what order its arguments are evaluated, its name should not resemble "sequence." Perhaps it could be renamed to something like unsafeForce. pseq, then, would be renamed to unsafeSeq.