Use doc_comment/4 instead of comment_hook/3

The former plays nicely with PlDoc while the latter causes mischief.
This change should also make it possible to reuse other code from
PlDoc in our implementation.

Closes #8
Reuse structured_comment from pldoc_process

Now that mavis cooperates with PlDoc, we can reuse PlDoc's code for
parsing structured comments rather than copying and pasting it into
our module.
Sort use_module lines
Don't create type assertions for the 'any' type

The 'any' type succeeds for all values.  There's no need to add this
type assertion during development time since it can't possible throw
an exception.  Having the(any, Foo) also clutters listing output.
Bump version