Add new interface for tagging assistants.

[Issue: #330]
Tag assistants in core/ast.

[Issue: #330]
Type constraint error with unary minus, fixes #380
Updates to test results related to generation of the if-expression
Added test revealing problem with generation of the if-expression
Added transformation for the if-expression
Update to test result related to generation of the if-expression
Remove the build status tag from the readme

(Given that it non-deterministically fails due to dependency downloads)
Removed synchronised keyword that should not be there
Updated test results related to generation of quotes
Changed the way quotes literals are generated such that their ordering can be controlled.
Merge remote-tracking branch 'origin/development' into ldc/quickies
Rename assistant interface to AstAssistant.

New name is more descriptive.

[Issue: #330]
Add interface marker to POG assistants.

[Issue: #330]
Add interface marker to interpreter def and exp assistants.

[Issue: #330]
Delete hollow assistant.

[Issue: #330]
Tag interpreter pattern assistants.

[Issue: #330]
Tag interpreter stmt assistants.

[Issue: #330]
Delete unused assistant.

[Issue: #330]
Delete vdmj compatibility tests

Not used anymore.

[Issue: close #377]