Revisit the Changes spec

[haarg]

The current spec for change logs is so loose that pretty much anything is a "valid" changelog file. This makes it hard to interpret changelogs. Since the spec is so loose, many things that are valid according to the spec are unparseable by the code (at least not into any reasonable format).

The spec also doesn't match well with the existing changelogs in the wild. Different "group" formats are common, as well as deeper nestings than just one level of groups. The data model of the module doesn't support this, but that is easy enough to fix.

I don't think treating it as a "spec" is really very useful.