Add pingpongseq()

[aidalgol]

lume.pingpongseq(max)

Returns a coroutine that continuously yields the next item in the sequence

local coro = lume.pingpongseq(3)
coro() -- Returns 1
coro() -- Returns 2
coro() -- Returns 3
coro() -- Returns 2
coro() -- Returns 1
coro() -- Returns 2
-- ...