Specialize traces

Currently, the ASTs represented by our builders form annotated binary trees, and as such, having traces be either Trace::Left or Trace::Right is okay. However, I will need more complex traces to represent builders once let bindings and pattern matches are added. As such, I believe it would be best to make the Trace enum type contain specific constructors to each builder shape, like AbsLeft, AbsRight. This will allow one to easily add new builders such as let, for which traces would be LetType, LetDef and LetBody.