Bug in `Shrink.list` skips singleton?

I was adding reports to some of the challenges in [The Shrinking Challenge](https://github.com/jlink/shrinking-challenge), namely [Length List](https://github.com/jlink/shrinking-challenge/blob/main/challenges/lengthlist.md).

Hedgehog was [not finding the optimal shrink](https://github.com/jlink/shrinking-challenge/blob/main/pbt-libraries/hedgehog-fsharp/challenges/length_list.md), so I took a look at `Shrink.list`.

It appears to skip over the singleton case?

```fsharp
#r "nuget: Hedgehog, 2.0.3"

open Hedgehog

for xs in Shrink.list [1; 2; 3] do
printfn "%A" xs

// []
// [2; 3]
// [1; 3]
// [1; 2]
```

Start by running the reported F# example against Hedgehog 2.0.3 and inspect the Shrink.list entry point. Compare its output with the Length List challenge report; done means the singleton shrink case is handled correctly and the relevant challenge result is no longer suboptimal.

Written by the indexing model from the issue text.