Issues with the current ascend extension interface

[timholy]

Having recently written tests for SnoopCompile's extension, and a draft new extension for JET, I decided it was time to document the extension API. In #577 I had already noticed one potential issue, but the experience also led me to note other issues:

• callstring (which calls method and specTypes) gets applied to both the original input object (or instance(obj)) and the edge items that come from iterating backedges(obj). This prevents edge from just being a state object that would let you extract the actual new node.
• there isn't a very clear delineation between "node data" and "node with tree structure"
• there isn't a good way to do "initialization" of the object, short of Cthulhu.treelist(userobj::ObjType) = Cthulhu.treelist(process(userobj)). But process should instead be an API call used after entering treelist.