Use the full AST in MessageContext

[stasm]

The runtime AST is a perf optimization designed specifically with the JavaScript frontend in mind. Do you think C# needs it as well?

For reference, we're actually thinking of dropping it in favor of the full AST, provided the runtime parser can keep its current performance: https://github.com/projectfluent/fluent.js/issues/220.

The Python implementation of MessageContext (currently a WIP) also uses the full AST: https://github.com/projectfluent/python-fluent/pull/67.

[blushingpenguin]

No, I don't think it needs it, it actually doesn't make a lot of sense in C#. I will eventually change it over to work with the full AST as you propose (however given it's effectively an implementation detail then it shouldn't affect using the library). Thanks for the suggestion!