Modularize the rewrite rules (move out those that don’t produce semantically equivalent code)

[julienrf]

So that users can have fine-grained control on which rules to apply if they want to. In particular, one possibly controversial migration is the replacement of Stream with LazyList, since they can produce different behaviors. We should isolate the “Stream => LazyList” rule (we can just name it StreamToLazyList).

See the related discussion here: scala/collection-strawman#528 (comment)