Current jvpp generator parses JSON api representation into dict of complex tuples/lists.

The dict is then modified by various components, which makes code difficult to mantain.

Use explicit immutable object model instead.

Type generation is tightly coupled with current approach, so probably needs to be rewritten along the way.

Not covered:

• integrate with vappigen plugin (VPP-1154) or vapi parser (VPP-1155)
• use better templating engine (VPP-1190),
• improvements of generator structure (e.g. VPP-1186)