Reexport desugaring and verification of typing rules

To write an extension it is now sufficient to import
extension.Declaration and possibly extension.TypeOf.

(Plus some syntax stuff like org.sugarj.languages.Fomega and concretesyntax. ...).
Cleanup: let and annotated let extension now use extension.Declaration
Cleanup: match pair now also used extension.Declaration
Added a strengthening clause to parallel reduction

This is useful for extensions where a C |- T ~> T' is
required in a context C;D where D are only expression
variable bindings. MatchPair is such an example.
Cleanup
An alternative desugaring using let