[stdlib] Add vectorized Span iterator

[martinvuyk]

Add vectorized Span iterator

Examples:

var iterator = iter(Span(List[UInt8](1, 2, 3, 4, 5, 6, 7, 8, 9, 10)))
var has_a_10 = False

for v in iterator.vectorized():
    has_a_10 |= any(v == 10)

for s in iterator:
    has_a_10 |= s[] == 10

print(has_a_10) # True

[martinvuyk]

I think this might be better as a List[T1].map(func: fn (T1) -> T2) -> Iterator[T2] API