Remove Radix::Result#key usage in the codebase

No longer expose internal functionality used only in tests. Reduce the
exposed elements of `Radix::Result` to focus on payload only.

Also remove the associated array used to collect all the traversed nodes
when performing the lookup.

NOTE: this is a breaking change

spec/radix/result_spec.cr

@@ -21,37 +21,6 @@ module Radix
       end
     end
 
-    describe "#key" do
-      context "a new instance" do
-        it "returns an empty key" do
-          result = Result(Nil).new
-          result.key.should eq("")
-        end
-      end
-
-      context "given one used node" do
-        it "returns the node key" do
-          node = Node(Symbol).new("/", :root)
-          result = Result(Symbol).new
-          result.use node
-
-          result.key.should eq("/")
-        end
-      end
-
-      context "using multiple nodes" do
-        it "combines the node keys" do
-          node1 = Node(Symbol).new("/", :root)
-          node2 = Node(Symbol).new("about", :about)
-          result = Result(Symbol).new
-          result.use node1
-          result.use node2
-
-          result.key.should eq("/about")
-        end
-      end
-    end
-
     describe "#use" do
       it "uses the node payload" do
         node = Node(Symbol).new("/", :root)