WIP: [#164] Adding FA2 spec, Integrational tests, STM tests.

Description

Work in progress MR that adds FA2 spec to morley-ledgers package and the tests for FA2 to morley-ledgers-test package.

As of now, it contains..

1. Integrational tests have been added to cover the functionalities of a FA2 contract which support a single token type, and does not have an external transfer_hook.

2. Types/Parameter for the FA2 contract

3. Some STM tests for the FA2 functionalities have been included. But it requires further work.

4. A Sample implementation of an FA2 contract, in Indigo. Right now only the transfer entrypoint and balance_of entrypoints are implemented.

Related issue(s)

Resolves #164 (moved)

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

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