[#185] update ManagedLedger implementation

Description

Modifies ManagedLedger implementation, in particular:

1. avoid treating from == to as a special case in transfer
2. store approvals in a separate big_map from ledger

Related issue(s)

Resolves #185 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

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  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

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  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

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