Fix non-iteratrable input of yieldEach

[Yi2255]

1. The input of yieldEach must be an iterable object, so separate yieldEach from YieldGenerator and use it as a separate code generator which requires iterable object.
2. For other code generators that need to use yieldEach, get random objects from the context, and then put those objects into an array as the input to yieldEach.

[saelo]

Thanks a lot!