added ability to add receptors as the surface of trees and have the tree free code know about them so they can free them
added in structure definition array for built in symbols so all sys and test symbols now have a structure.

Note: some of the symbols are set to have NULL_STRUCTURE, because I don't know how to deal with them yet.
also, reorganized the structure of a structure def so that no data is stored at a leaf node any more.
more work on giving all sys and test symbols valid structures, included refactoring _t_dump to print based on structure, not symbol