Improve slicing

Currently our slice definition looks as follows:
```rust
pub fn slice>(&self, ranges: &[D]) -> anyhow::Result {
///...impl...
}
```

This is very user hostile, as the user must provide a homogeneous collection of ranges.
```rust
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
```rust
let y = x.slice(&[.., 0..6, ..]);
```

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

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

```rust
let y = x.slice(s![.., 0..6, ..]);
```
This API gives the illusion of heterogeneous collections, which is what we want.

Start at the Tensor::slice API shown in the issue and study how ndarray's s! macro represents heterogeneous ranges. Define the macro-based slicing interface and verify that examples such as s![.., 0..6, ..] work while preserving the intended slicing behavior.

Written by the indexing model from the issue text.