convert BinaryTree to slightly more canonical Julia form

With no node mutation, it is unclear why this is mutable (which costs
some extra loads), though the recursion forces this to the heap
regardless. But singleton struct Empty needs to be non-mutable now for
correctness

src/shootout/binary_trees.jl

@@ -4,17 +4,17 @@
 #
 # Ported from an OCaml version
 
-abstract type BTree end
 
-mutable struct Empty <: BTree
-end
+struct Empty end
 
-mutable struct Node <: BTree
-    info
-    left::BTree
-    right::BTree
+struct Node{T}
+    info::T
+    left::Union{Node{T}, Empty}
+    right::Union{Node{T}, Empty}
 end
 
+const BTree{T} = Union{Node{T}, Empty}
+
 function make(val, d)
     if d == 0
         Node(val, Empty(), Empty())