Double rounding issue with fromRational

Currently, `fromRational :: Rational -> Half` is implemented as a composition `(toHalf :: Float -> Half) . (fromRational :: Rational -> Float)`. Unfortunately, this implementation is not correctly-rounding (i.e. round to nearest).

For example, consider `0x1.005fff8p0`. The nearest values expressible in `Half` are `0x1.004p0` and `0x1.008p0`, so `0x1.004p0` should be returned by `fromRational 0x1.005fff8p0 :: Half`. However, the current situation is:

```haskell
ghci> showHFloat (fromRational 0x1.005fff8p0 :: Half) ""
"0x1.008p0"
```

Fixing this is easy, as `base` exposes [`fromRat :: RealFloat a => Rational -> a`](https://hackage.haskell.org/package/base-4.20.0.1/docs/Numeric.html#v:fromRat), which should be correctly-rounding.

Of course, neither [Haskell 2010](https://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-1350006.4) nor current [GHC base](https://hackage.haskell.org/package/base-4.20.0.1/docs/Prelude.html#v:fromRational) specifies `fromRational` to be correctly-rounding, so this is not strictly a bug. Nevertheless, I think the behavior should be documented if we are not going to "fix" this.