Support the arguments object

[benjamn]

In order to support generator functions that access the arguments object, we need to save a reference to the arguments of the outer function in a freshly-named variable, and then rename all free references to arguments in the inner function.

It's tempting to just store the arguments object as context.args, but I'm not sure that would preserve the parameter aliasing behavior of arguments (i.e., if you modify arguments, you're also modifying the actual parameters passed to the function, and vice-versa).