Don't leave variables bound after function execution :(

lib/chervil/ast/function.rb

         raise "Expected #{params.size} arguments but received #{args.size}"
       end
 
+      env = ::Chervil::Env.new(env)
+
       @params.zip(args).each do |key, value|
         env.set(key.name, value)
       end

spec/interpreter_spec.rb

       interpret('(define (plus-one x) (+ x one))', env)
       expect(interpret('(plus-one 2)', env).first).to eq(3.0)
     end
+
+    it 'doesn\'t leave variables bound after function execution' do
+      env = Env.new
+      interpret('(define (plus-one x) (+ 1 x))', env)
+      interpret('(plus-one 5)', env)
+      expect(env.get('x')).to eq(nil)
+    end
   end
 end