Support variant serialization/deserialization for the latest Ergo runtime

[jeromesimeon]

Is your feature request related to a problem? Please describe.
Ergo 0.20.* relies on a few changes to the Concerto serialisation that have to be consistent with the Ergo JavaScript run time. The changes include:

• Alternative representation for enums consistent with the new typed enums in Ergo
• Alternative representation for Integer and Long values to ensure proper printing in toString and in the templates support
• Additional option for serialisation of relationships

This should be controlled by a proper option on the serialiser and ensure backward compatibility when that option is turned off (which should be the default).

Describe the solution you'd like
Fold changes for that serialization from the Ergo 0.20 branch into Concerto, which avoid much code duplication and provides for better and more systematic testing.

Additional context
This is part of a core refactoring for the 0.20 release. This change will also facilitate review and discussion for improvements in future releases.

[jeromesimeon]

Initial code for this can be found in accordproject/ergo@500b0f7

PR which supports this new option and passes all the tests is available at:
#111

[jeromesimeon]

Fixed in #111