feat: small Structs and Messages update

[novusnota]

• Instantiation description
• Trailing comma
• Field punning
• .toCell() extension function
• Cross-link to Expressions#instantiation

Note, that .toCell() adds links to non-existent yet Reference section, but those links will be eventually correct once #206 is completed (quite soon!).

Closes #210 (field punning) — main target of this PR Closes #212 (.toCell() function) Closes #154 (just a reference link)