Better memory layout by moving len before xs

[JakkuSakura]

Hi, I noticed that len is arranged after xs

pub struct ArrayVec<T, const CAP: usize> {
    // the `len` first elements of the array are initialized
    xs: [MaybeUninit<T>; CAP],
    len: LenUint,
}

In C's world, it's a convention to move length before an array, like

#[repr(C)]
pub struct ArrayVec<T, const CAP: usize> {
    len: LenUint,
    // the `len` first elements of the array are initialized
    xs: [MaybeUninit<T>; CAP]
}

This provides a few nice features:

• better locality. First, read the length, then read the element one by one. This should be trivial though
• Easier to implement tail arrays if were to put on the heap
• https://github.com/antirez/sds Simply Dynamic String uses a similar layout

[Dushistov]

It would be also nice, if LenUnit will be generic parameters,
so for CAP < 256 it would be possible to use LenUnit = u8.
The memory layout would be even better.

[JakkuSakura]

sure, I'll make len type a generic parameter with default type

I'll also add a reference type to ArrayVec

[YuhanLiin]

The ordering of fields in the default representation (repr(rust)) is unspecified, so the length field might already be ordered before the data. Of course this depends on the compiler's discretion.