Feature: Add Vec::spare_capacity_mut

[survived]

It'd be great to have spare_capacity_mut method, similar to what alloc::vec::Vec has.

impl<T, const N: usize> Vec<T, N> {
    pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] { /* ... */ }
}

Having access to spare capacity enables certain use-cases, for instance, I wanted to securely zeroize a vector, and to do that I need to zeroize the spare data as well.