QVT14 — QVTo: Rework ImperativeOCL to compose rather than delegate EssentialOCL

Issue 13082 - http://solitaire.omg.org/browse/QVT13-8 identifies the unsound relationship between ImperativeOCL and EssentialOCL.

It offered a simple textual fix which was exploited to resolve the original issue.

It also suggests a harder rework to establish modeling integrity. This issue forks off the rework not resolved by the original issue.

(B) - (Major rework.) Rework the abstract syntax to reuse OCL
expressions by composition rather than by inheritance.
Imperative expressions ( => rename to 'statements' ) then may
contain sub-statements and OCL expressions; OCL expressions
are reused unchanged from the OCL spec (no imperative
sub-expressions, no side-effects).
These issues have been discussed on the MoDELS 2008 OCL Workshop,
more details can be found at
http://www.fots.ua.ac.be/events/ocl2008/PDF/OCL2008_9.pdf

(Since this is a breaking structural change, it is unlikely to happen before QVT 2.0)