Add tests for Concatl and Concatr, code for naive cases

Not implemented: ftree.Concatr(ftree)
Add concat support for ftree (+) ftree
Add test for empty.Popr()
Extract Concatr's helper functions into separate functions to improve readability

Rename app3 to glue, which is more descriptive. Also add comments for app3 and nodes.
Improve use of iterator functions, remove dead code
panic if an attempt is made to generate a node from a single item
Merge pull request #10 from alaroldai/concat

Concat