Support sequence point, add permissions, and update prelude.

Both the operational and axiomatic semantics are extended with sequence points
and a permission system based on fractional permissions. In order to achieve
this, the memory model has been completely revised, and is now built on top
of an abstract interface for permissions.

Apart from these changed, the library on lists and sets has been heavily
extended, and minor changed have been made to other parts of the prelude.