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>

(cherry picked from commit [c1035d51edaac2f388a0630e2ad25391e7e3c1ab/ghc])

Differential Revision: https://phabricator.haskell.org/D25
Fixup c1035d51e to behave more like in GHC 7.6

The fix in c1035d51e (addressing #9231) was done under the assumption
that `Data.Fixed` is used only with power-of-10 `resolution`. This
follow-up fixup changes the behavior to be more consistent with the
previous behavior in GHC 7.6

For instance, for the following `B7` resolution

   > data B7
   > instance HasResolution B7 where resolution _ = 128

After this patch, the following behavior is now observable:

   > 1.070 :: Fixed B7
   1.062
   > 1.062 :: Fixed B7
   1.054

   > read "1.070" :: Fixed B7
   1.062
   > read "1.062" :: Fixed B7
   1.054

This doesn't provide the desirable "read . show == id" property
yet (which didn't hold in GHC 7.6 either), but at least `fromRational`
and `read` are consistent.

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

(cherry picked from commit [ec550e8f951e50fb91c89389e2e77a3358079c3a/ghc])
Bump to 4.7.0.1 and update changelog.md

Fwiw, the version bump should have been performed together with the first
change after 4.7.0.0

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