Evaluate identifier referents

lib/chervil/ast/identifier.rb

     end
 
     def evaluate(env)
-      env.get(@name)
+      env.get(@name).evaluate(env)
     end
   end
 end

spec/ast/function_spec.rb

             )
           ],
         ).call(
-          [5.0]
+          [AST::Number.new(5.0)]
         )
       ).to eq(25.0)
     end

spec/ast/identifier_spec.rb

       value = AST::Number.new(5.0)
       definition = AST::Definition.new(identifier, value)
       definition.evaluate(env)
-      expect(identifier.evaluate(env)).to eq(value)
+      expect(identifier.evaluate(env)).to eq(5.0)
     end
   end
 end

spec/interpreter_spec.rb

       env = Env.new
       interpret('(define x 5)', env)
       expect(env.get("x")).to eq(AST::Number.new(5.0))
-      expect(interpret('x', env).first).to eq(AST::Number.new(5.0))
+      expect(interpret('x', env).first).to eq(5.0)
     end
   end
 end