[TZI-7] Update contracts to reflect spec changes

Description

Problem: FA1 featured "UnsafeLedger" but will be changed to be compatible with the more or less adopted FA1.2 standard. In the scope of these changes, "UnsafeLedger" will be removed, and "AbstractLedger" will take its place.

Solution: Rename "UnsafeLedger" to "AbstractLedger", change the interface to accept a :from parameter. Add a check that :from is equal to sender prior to transfer.

Related issue(s)

https://issues.serokell.io/issue/TZI-7

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
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