coroutine/generator: introduce buffered generator for batch processing

before this change, we only have an unbuffered generator
implementation, where the producer and consumer operated in a
"ping-pong" fashion. elements were produced and consumed one at a time.

in this change, we introduce a buffered variant of the generator,
which allows producer to yield a range of elements in batch. it
also enables consumer to process multiple elements without suspension.

the benefits are:

* improved efficiency for batch operations
* reduced context switching between producer and consumer
* enhanced flexibility in element production and consumption patterns

Signed-off-by: Kefu Chai <[email protected]>