The unpac monorepo manager self-hosting as a monorepo using unpac

Prepare to release v1.15.4

Signed-off-by: David Scott <dave@recoil.org>

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CHANGES.md
··· 1 + ## v1.15.4 2 + 3 + - Fix build on OCaml 4.02 and fix SHA equality bug by @djs55 (#61) 4 + 1 5 ## v1.15.3 2 6 3 7 - Fix build on OpenBSD by @kit-ty-kate (#58)