`alter` needs to be exported from `Data.Map.Ordered.Strict`

The release notes for 0.2.4 claim a strict variant of `alter` was added, but it is not there... looking at the commits, I can see that `alter` was indeed added to `Data.Map.Ordered.Strict`, but it was not added to the export list.

I could make a PR for this soon, just wanted to write it down before I forgot. 😄 