treemap: improve the lazy iterator

* replace the dual next() and get() calls with a single next() function
* drop one of the pointer members from the struct
* add a method for using the lazy iterator with a for loop
auto merge of #5123 : thestinger/rust/treemap, r=nikomatsakis

* replace the dual next() and get() calls with a single next() function
* drop one of the pointer members from the struct
* add a method for using the lazy iterator with a for loop