Resolve "From SLL to DLL"

This is a rather special exercise, so bear with me. 😅

This adds an exercise to transform a SLL into a DLL, the idea as discussed in #24 (closed) is to give them the SLL as template solution and guide them towards a DLL.

So the way this exercise works is that I have the template solution (SLL) in the library, while the DLL is in the solution. I've tried with the with remove_block():, however we do not only add stuff to the SLL to get to DLL, we also do the opposite 😅

So a big concern in this exercises is mainly, do we give enough guidance to the students, or perhaps is it too much guidance?

For the tests, I have slit it up into two parts.

• Tests for any linked list behaviours, these will pass both for the SLL and DLL, these are all visible tests. Mainly there for students to see that they aren't breaking any default behaviour.
• Tests for DLL behaviour, these are the previous tests but adapted with DLL behaviour (checking for previous and tail), a small one is included in the visible tests.

Closes #24 (closed)