Clean up REPL

bin/chervil

 while line = Readline.readline("> ", true)
   lexer = Chervil::Lexer.new(line)
   tree = Chervil::Parser.new(lexer).parse
-  puts Chervil::Interpreter.new(tree, env).interpret
+  results = Chervil::Interpreter.new(tree, env).interpret
+  results.each { |result| puts result unless result.nil? }
 end

lib/chervil/ast/definition.rb

 
     def evaluate(env)
       env.set(@name.name, @value)
+      nil
     end
   end
 end

lib/chervil/ast/function.rb

       @params == other.params && @body == other.body
     end
 
+    def evaluate(env)
+      self
+    end
+
     def call(args)
       unless @params.size == args.size
         raise "Expected #{params.size} arguments but received #{args.size}"
 
       current_expr
     end
+
+    def to_s
+      "<function>"
+    end
   end
 end