Add len_u16 and len_u8 features to change the type of LenUint

[WuBingzheng]

Add len_u16 and len_u8 features to change the type of LenUint.

Using u8 as LenUint saves 3 bytes. This is useful for many really small strings.

[GnomedDev]

This should not be a feature, as features should be entirely additive. I would recommend making it a generic, maybe?

[WuBingzheng]

I do not know how to make it a generic. Use the num trait?

pub struct ArrayVec<T, L, const CAP: usize>
where L: num::Num
{
    xs: [MaybeUninit<T>; CAP],
    len: L,
}