[#16] Implement AbstractLedger in Indigo

Description

AbstractLedger was partially implemented in Indigo, but main structure of it still uses Lorentz directly. Here I port it fully to Indigo.

I haven't touched tests, existing AbstractLedger tests should cover the changes.

Related issue(s)

Resolves #16 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

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    • README
    • Haddock
    • docs/
  • I updated the changelog if my changes are externally visible.

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