Add newline before two `/Since: 4.7.0.0/` occurences

See also `/Since: .../` convention described in
6368362f4486226dee9d54665b4fdffde24e56f1

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Add Hackage-supported `changelog.md` file

The contents has been ripped from the release notes. I've used markdown
as this may allow for automatic conversion to docbook XML in the future,
should we choose to maintain the `base` changelog in a more human-friendly
markup format and closer to the `base`-source tree.

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