Fix regression in Data.Fixed Read instance (re #9231)

`convertFixed` operates under the wrong assumption that
`Data.Fixed.resolution` returns a base-10 exponent `e`, whereas it
actually returns the power-of-10 value `10^e`.

So while the code (that was introduced in 5f19f951d8 / #7483) happens to
compute a correct result by coincidence, it allocates huge integer values
in the process (i.e. `10^(10^e)`) which cause long computations which
eventually result in out-of-memory conditions for `e`-values beyond
`Data.Fixed.Milli`.

A proper long-term fix would probably be to extend the `HasResolution`
type-class to have a 2nd method providing the `e` value.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>

Differential Revision: https://phabricator.haskell.org/D25