engage mixins before considering a tree pre-finalized

For the sake of `beforeFreeze` callbacks, _tree pre-finalizes a tree,
executes callbacks, and then finalizes the tree. To consider the tree
pre-finalized, the mixins need to be present. This takes care of that.
built files for previous commit
v0.3.1: make explicit finalization stages

mixins were not engaged before dispatching callbacks with the
"pre-finalized" tree. This proved an issue in one instance, and seems
generally wrong. Now a pre-finalization stage is explicit, with mixins
and ancestry setup before calling callbacks.