Define `vsepHard`

Define
```
vsepHard :: [ Doc a ] -> Doc a
vsepHard = concatWith (\x y -> x <> hardline <> y)
```

This is a missing function. We have n-ary concatenation functions for various separators: non-breaking spaces, breaking spaces, lines, softlines... but not hardlines.

`vsepHard` is useful when you have things that you just _never_ want to lay out on a single line. For example, in a programming language you might define let-bindings like so
```
let
x = 1
y = 2
in t
```

I always want the `x` and `y` bindings to be on separate lines. A more sophisticated version might use `flatAlt` to use a different syntax when `group`ed, but I think the simple approach of "just always break" is often useful.