Add `/Since: 4.7.0.0/` to `MkFixed` constructor

As it's been exposed only since
05c03d850a7da7bd708e9a9da7de2b9ba36b6041

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Update `changelog.md` to contain more changes

Some important changes might still be missing from the changelog.

Also, `base.cabal` is updated to include `changelog.md` in the
`cabal sdist` tarball.

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