Proposal: Add singleton function to Data.List module

[Taylor Fausak]

I originally made this suggestion on GitLab, but I was told to make it here instead.

https://gitlab.haskell.org/ghc/ghc/issues/17042

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# Add list singleton function

## Motivation

Sometimes it is convenient to have a function to wrap an element in a list. There are many ways to do this already, but none of them are as clear as a separate monomorphic function.

- `pure`: Polymorphic, works for any `Functor`.
- `pure @[]`: Noisy, requires `-XTypeApplications`.
- `(: [])`: Subjectively ugly.
- `(\x -> [x])`: Syntactically noisy.

This Twitter thread includes some additional commentary: https://twitter.com/taylorfausak/status/1159264862247280640

## Proposal

I would like to add a `singleton` function to `Data.List` that mirrors the `singleton` function for other containers: https://www.stackage.org/lts-14.0/hoogle?q=singleton

``` hs
singleton :: a -> [a]
singleton x = [x]
```

Other Haskell-like languages include this function:

- PureScript: https://pursuit.purescript.org/packages/purescript-lists/5.4.0/docs/Data.List#v:singleton
- Elm: https://package.elm-lang.org/packages/elm/core/latest/List#singleton