Add uninit

[ajz34]

This will be a major code refactor, probably an API breaking change.

This PR will focus on these topics:

• Add uninit, which works like Rust's MaybeUninit.
• Add assume_init, which works like assume_init but on the whole tensor instead of an element.
• Change the internal functions that works with empty_impl to uninit_impl.

Fixes #7

[ajz34]

The correct way to use uninitialized vector, fill value of uninitialized tensor, then transform that to be a usual tensor, can be some code like:

#[test]
fn correct_code() {
    use rstsr_core::prelude::*;
    use rstsr_core::prelude_dev::*;

    // Generate a tensor of uninitialized vectors
    // Note that `MaybeUninit` will not call the destructor of `Vec<usize>`.
    let mut a: Tensor<MaybeUninit<Vec<usize>>> = rt::uninit([12]);
    // Some code to initialize the tensor elements
    a.iter_mut().enumerate().for_each(|(idx, x)| {
        *x = MaybeUninit::new(vec![idx]);
    });
    let a: Tensor<Vec<usize>> = unsafe { rt::assume_init(a) };
    println!("{:?}", a);
}