Allow tuples to be compared with == operator

### What is wrong?

We currently can't easily compare tuples with the `==` operator. E.g. the following doesn't work.

```
if get_tuple() == (1, 2) {
...
}
```

Instead what we have to do is:

```
if get_tuple() == (1, 2) {
...
}
let (val1, val2): (u8, u8) = get_tuple()
if val1 == 1 and val2 == 2 {
}
```

### How can it be fixed

~~We can probably tackle that at MIR lowering~~

Make `PartialEq` and `Eq` traits possible and use them for this.

Start by tracing tuple equality through the compiler and the MIR-lowering area mentioned in the issue, then inspect how the PartialEq and Eq traits are represented. The issue considers trait support the intended direction; done means tuple values can be compared with == as shown and the behavior is covered by relevant tests.

Written by the indexing model from the issue text.