Add a lazy `Reader`

[Baptouuuu]

Problem

When translating the DOM representation of this page (1Mo) to the structures from this package it no longer fit in memory.

Possible solution

Add a lazy Reader that recompute the translation each time it is accessed.

However this may not be possible due to the fact that a Reader must return a Maybe.

[Baptouuuu]

Alternatively creating a parser in order to stream the parsed xml could be interesting in order to read files that event the string representation doesn't fit in memory.

Maybe an inspiration could be taken from here https://github.com/carlosplanchon/xmlstreamer