Allow multiple values to be returned while iterating

[schilkp]

When using the iterator, getting more than 1 byte at a time requires a lot of overhead.

Ideally something like:

ws2812b_iter_next_n(ws2812b_handle_t *ws, uint8_t *buf, int len)

could reduced the overhead (mostly iteration index to actual state mapping) when providing multiple values.

[schilkp]

fill_buffer should probably just fall back on a call to this.

[schilkp]

keep a iter_next function with same semantics (i.e. return single value, not through pointer).

That can of course fall back on the above mentioned function.

[schilkp]

Maybe move the iterator state outside of the handle struct?
-> Create Handle
-> Create Iterator(s)
-> Use iterator(s)