Minimum edit distance between sentences

[heatherleaf]

The backend should compute the distance between the current sentence and the goal, and send it to the frontend (which can use this when calculating scores etc)

Suggestions for distance measures:

• very simple: Levenshtein distance between the "semantic" strings (i.e., the lists of abstract function nodes that is already used by muste/src/Muste/Menu/Internal.hs to calculate insertions)
• simple: Levenshtein distance between the linear representation of the abstract trees
• not-so-simple: Tree edit distance
• complex: use the solution path calculated by Analysing exercises to get the shortest path #58

[heatherleaf]

the "simple" solution was implemented in aa254a7