Added skeleton implementation of a B-tree with a few bells and
whistles.  No major functionality added yet (such as insertion and
removal).
Re-arranging some things in btree.rs to accommodate testing.
Addressed style concerns in btree.rs and added it to the lib file.
Added documentation within btree.rs.
auto merge of #10303 : niftynif/rust/master, r=catamorphism

r? @catamorphism
First contribution; contains struct definitions and some useful traits.  Can possibly be improved by removing the dependence on the Clone trait.  This is intended to be the beginning of a long-term project.