speed up javascript generator

[tocque]

Await is not necessary in generator.

[hanabi1224]

This is coroutine benchmark, usage of generator aka simi-coroutine is not allowed

[lohaswinner]

This is coroutine benchmark, usage of generator aka simi-coroutine is not allowed

But originally coroutine execution should not return at the end of body. So usage of yield value is the feature of generator. AsyncGenerator without await in the body doesn't have any difference from basic generator.