Add new interface for tagging assistants.

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

[Issue: #330]
Type constraint error with unary minus, fixes #380
Remove the build status tag from the readme

(Given that it non-deterministically fails due to dependency downloads)
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]
Merge remote-tracking branch 'origin/development' into ncb/development
Merge remote-tracking branch 'origin/pvj/main' into ncb/development
Merge remote-tracking branch 'origin/pvj/main' into ncb/development
Problem with object reference compares in postconditions, fixes #388
IO`freadval uses the default character encoding, fixes #391
Merge remote-tracking branch 'origin/ncb/development' into ncb/development
Merge remote-tracking branch 'origin/ncb/development' into pvj/main