Improve slicing

Currently our slice definition looks as follows:

    pub fn slice<D: std::ops::RangeBounds<usize>>(&self, ranges: &[D]) -> anyhow::Result<Tensor> {
        ///...impl...
    }

This is very user hostile, as the user must provide a homogeneous collection of ranges.

let y = x.slice(&[0..5, 0..6, 0..7]);

This is very annoying, as you may only care about one of the dimensions, it would be better to have something like

let y = x.slice(&[.., 0..6, ..]);

Unfortunately, this doesn't work, because the collection is now heterogeneous (i.e is made up of 2xRangeFull and 1xRange).

Therefore, we need to use a macro, much like ndarray.

let y = x.slice(s![.., 0..6, ..]);

This API gives the illusion of heterogeneous collections, which is what we want.