[#333] Fix encoding of addresses with %default entrypoint

Description

Problem: The SELF %default instruction results in an address in the format "address%default". Previously, PACKing this address would result in a different bytes result than for the equivalent "address" with the implicit default entrypoint.

Solution: Canonicalize the %default entrypoint before encoding it.

Related issue(s)

Resolves #333 (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:
    • 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.