Support ES5 getter / setter

[bendlas]

Those can't be transpiled away, so it would be nice, if jwacs was able to deal with them. Options include:

• Just CPS the etter function and let it trampoline
• Leave the etter function alone
• Instrument all candidate property accesses to the etter

Letting it trampoline doesn't seem like a good option, because it introduces unexpected pitfalls in case of asynchronous continuations, so just passing the etter through seems like a good first step.

Instrumenting all the property accesses would potentially incur a large overhead, especially in case of computed property names.