[#329] Fix 'SELF' instruction packing

Description

Problem: 'SELF' instruction has incorrect packing in case of field annotation presence.

Solution: Fix it.

Related issue(s)

Resolves #329 (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. Currently, serialization tests for instructions are based on lambdas and SELF cannot appear in lambdas, so it's a bit nontrivial to add some tests into existing serialization tests. However, this fix will be tested as a part of !549 (merged).

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

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