Specify a `Size` without unit (e.g. for `line-height`)

It would be nice to be able to specify a number without a unit for `Size Value`.
I mean, does it makes sense to make a PR with such function?
```
num ∷ Number -> Size Value
num = Size <<< value
```

So it would be possible to write:
```
lineHeight $ num 2.0
```

Not sure if this looks good though :joy: 