Refactor exhaustiveness checking

[giacomocavalieri]

This is quite big change refactoring the pattern matching compilation code.

• I've changed a lot of names here and there to be a bit more descriptive and hopefully help understand what is going on in the code
• I've also added quite a few of documentation comments to try and explain all the terminology used and hopefully make things clearer to skim through
• After removing unused code, a couple of files were small enough to be deleted and just be made part of the exhaustiveness.rs one. Most notably, Pattern was moved there
• While the decision tree has changed in its shape, the Match data structure hasn't changed much. The code consuming the result of the compilation hasn't been touched by this refactoring. However, I've tried to make the public facing api easier to use; as a result, I think that now the code in type_/expression.rs is a lot nicer

After this refactoring gets merged I will move on to deal with compilation of string patterns (and maybe bit arrays as well!), and finally we could start using the tree for code generation on the js target