Prepare v0.5.6.0 release

We need to bump the version and update the changelog before cutting a release:)

array.cabal

 cabal-version: >= 1.10
 name:          array
-version:       0.5.5.0
+version:       0.5.6.0
 
 -- NOTE: Don't forget to update ./changelog.md
 license:       BSD3