fix: handle context accesses following calls

[woodruffw]

WIP; These grammar changes mean the syntax tree itself is different, so I need to update the walk as well.

Fixes #339.

[woodruffw]

Some notes for myself:

• The Context node is now insufficiently expressive: an expression like fromJson(steps.runs.outputs.data).workflow_runs[0].id should be:

Context
  Call { fromJson args: Context { lit:steps, lit:runs, lit:outputs, lit:data } }
  lit:workflow_runs
  Index { 0 }
  lit:id