[#16] Implement AbstractLedger's transfer using Indigo

Description

Indigo is a higher level language than Lorentz, so it should be preferred for smart contract development. We want to demonstrate its usage and figure out its drawbacks. So it makes sense to port existing Lorentz contracts to Indigo.

We start with AbstractLedger as it is the simplest one. Here I only implement transfer entrypoint, because other entrypoints emit operation and it is not supported yet.

I had to add some things to Indigo to make it all possible

Related issue(s)

Resolves part of #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.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • README
    • Haddock
    • docs/

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.