PayloadParser refactor

Need to refactor a few more tests. Otherwise, this is done.
Continuing testing symbol referencing – this tests verifies that any additional content after a symbol reference is reported and ignored (and the reference is still expanded).

```
# Symbol [/]
+ Model

        { ... }

## Retrive [GET]
+ Request A
    [Symbol][]

    Report and ignore this text

+ Response 200
```