tests: Always draw a task for ParseContext

parse: Add function to format a ParseContext for display

parse: Add function to format a ParseErrorException for display

tests: Document parse error data structures

tests: Fix draw_maybe ratio ...

I wrote this code while really tired and of course an off by one error
slipped in.

tests: Use a proper values for structural ParseContext tests

src: Check for None in all __eq__ methods ...

This lets us finally compare parse error exceptions and contexts of
different shapes.

tests: Move parser error tests to its own file