Functions to convert DList to List at the end

[chshersh]

When users want to analyse resulting Fiasco and Result, they need to work with DList when they actually want to work with list. We need to provide some utilities for this.

A few options:

1. Create a function of type Trial e a -> ([e], Maybe a)
2. Create patterns FiascoL and ResultL which like the data type but with lists.
3. Reexport a function DList a -> [a]

[vrom911]

I would also add some filtering functions: fiascoErrors fiascoWarnings. What do you think?