Add test case for inter-module type/data family instances

These should show up in every place where the class is visible, and
indeed they do right now.

Signed-off-by: Mateusz Kowalczyk <fuuzetsu@fuuzetsu.co.uk>
Use a bespoke data type to indicate fixity

This deals with what I imagine was an ancient TODO and makes it much
clearer what the argument actually does rather than having the user
chase down the comment.
Strip a single leading space from bird tracks (#201)

This makes bird tracks in the form

> foo
> bar
> bat

parse as if they had been written as

>foo
>bar
>bat

ie. without the leading whitespace in front of every line.

Ideally we also want to look into how leading whitespace affects code
blocks written using the @ @ syntax, which are currently unaffected by
this patch.