Optimizer interface and sample implementation (#19)

Reviewed-on: Rubydragon/MetagraphOptimization.jl#19
Co-authored-by: Anton Reinhard <anton.reinhard@proton.me>
Co-committed-by: Anton Reinhard <anton.reinhard@proton.me>

src/node/print.jl

@@ -4,7 +4,7 @@
 Print a short string representation of the node to io.
 """
 function show(io::IO, n::Node)
-    return print(io, "Node(", n.task, ")")
+    return print(io, "Node(", task(n), ")")
 end
 
 """