Uses ‘mem_alloc’ to allocate a ‘Bstree’ and initialises it to
an empty bstree. Returns a reference to the allocated bstree, or a null
pointer if there was insufficient memory.
— Function: void bst_destroy (Bstree *bst)
Finalises the internal data structures of bst, and then frees
bst itself. bst must have been allocated by
‘bst_create’.