rewrite shootout-binarytrees.rs to match shootout directives

The old version didn't follow shootout's directives: no memory were
deallocated.  At the same time, parallelisation of the computation.

fix #2913
auto merge of #10145 : TeXitoi/rust/issues-2913, r=bstrie

The old version didn't follow shootout's directives: no memory were
deallocated.  At the same time, parallelisation of the computation.

fix #2913