AstFieldExpression is serving too many purposes:

1.  A variable reference when the path is of the form "$$var"

2. A field reference when the path is of the form "$x" or "$$var.x"

3. A nested field reference when the path is of the form "$x.y" or "$var.x.y"

These are semantically different, and will instead be represented using:

1. AstVarExpression
2. AstVarExpression + AstGetFieldExpression
3. AstVarExpression + AstGetFieldExpression + AstGetFieldExpression