Bug fixes to abstraction layer

This commit fixes a number of bugs in the abstraction layer:

- in `ModComposition.par`, `m1` and `m2` were used inconsistently.

- in `HighLevelSwitch_common.from_par` and `HighLevelSwitch_common.from_seq`, `List.fold_left` was wanted instead of `List.fold_right`

- in `HighLevelSwitch0x01.from_action` coercions frmo VInt were failing for parsed NetKAT expressions.

It also exposes a few more pretty printing functions from SDN_Types to aid debugging.