Allowed Vector Subsets during assignment

This is just a copy of this comment https://github.com/dgkf/R/pull/112#issuecomment-2054041873 as I don't want it to get lost in case the PR gets merged.

R has some edge-cases where subset-assignment has weird (and I image undefined) behavior:

```r
> x[c(1, 1)][c(1, 2)] <- c(99, 100)
> x
[1] 100 2 3 4 5 6 7 8 9 10
```

I think we should only allow vector subsets in assignment calls where the behavior is well-defined.
From the currently supported subsets, I think `Subset::Mask` and `Subset::Range` have this property, i.e. they can never produce a view where the same data is referenced via different indices of the resulting view.