Integrate `hnix-store-benchmark` from #26 and `hnix-store-dump` as executables

[sorki]

The thingie is pretty self-contained and doesn't even seem to depend on -core. The ReadonlyStore depends on it but that should also be a separate package it seems.

This would make dependencies cleaner and maintenance a bit easier.

CC @imalsogreg @domenkozar @sandydoo - I would add you all as maintainers on Hackage. Thoughts?

[sorki]

Could also include hnix-store-benchmark from #26 and hnix-store-dump as executables.

[sorki]

Could also include hnix-store-benchmark from #26 and hnix-store-dump as executables.

Since the thingie is a lib they should be guarded with flags like in -dbs cabal + buildable so they won't propagate to users PATH by default but enabled in dev env via cabal.project

[sorki]

When this lands on Hackage, the migration path for cachix is simple:

• add lower bound to hnix-store-core to require >= 0.8
• add hnix-store-nar