Version Groups and VersionMap trait

Added

• Added changelog
• The v attribute is an alias for the version attribute.
• Added the versioned group pattern with example

Updated

• The version attribute now requires an explicit index.
• The versions attribute now requires an entry for the current version.
• Use a trait for VersionMap instead of a HashMap<String, usize>

Removed

• The #[versions("Av1", "Av2")] syntax is not supported anymore, instead use the more explicit version
  #[versions(v(index = 1, type = "Av1"), v(index = 2, type = "Av2"))]