Opportunity for AST transformers design improvements in Kolasu 1.7

[alessiostalla]

There are a number of ways in which AST transformers could be improved that would break API compatibility and can't be done in Kolasu 1.5.

This is an umbrella issue for all of those, including:

• make the AST transformer itself stateless, passing a TransformationContext along the call chain; it will encapsulate all state, like issues, and potentially user-defined data to carry along; this could allow addressing Consider having a transformer being influenced by the context #202
• rename NodeFactory to something like TransformationRule and registerNodeFactory to registerTransformation
• other issues we could address: KolasuParser, assignParents and AST transformers #291, Experiment with transformers in Kolasu 1.6 #330, AST Transformer API issue with children set at construction #374, AST transformers in case of parse errors #395

[ftomassetti]

Thank you @alessiostalla . I added it to the Milestone for 1.7.0

[alessiostalla]

Other notable points to address:

• Exception handling
• Source propagation (we currently don't keep track of a possible Source in AST transformers, so any issue raised during transformations has a position without a source)