LeafCache is public but not implementable

[MFranceschi6]

Is your feature request related to a problem? Please describe.
I'd like to implement a custom LeafCache

Describe the solution you'd like
Have read access to LeafAST fields that need to be cached

Describe alternatives you've considered
Have LeafAST conform to Codable and have the field name public.
I suppose the effort to have LeafAST conforms Codable to not be too much problematic thanks to the introduction of the Codable enums in swift 5.5

Additional context
LeafCache is not implementable because LeafAST doesn't even have the field name public which is how LeafCache requires to store the leafAST.
It won't be enough to have it public because we don't have access to the others fields.