release: 0.8.1

[simontreanor]

Summary

• Bump version to 0.8.1 in all four tracked artifacts (Cargo.toml, VS Code extension, JetBrains plugin, Emacs mode).
• VS Code CHANGELOG entry for 0.8.1.

Ships the two fixes found dogfooding a real Pyfun program against 0.8.0:

• Assigning a module mut from inside an async block silently binds a local: the nested async def lacks the global #122 — a module mut assigned inside async/seq/result silently bound a local instead of the module binding.
• A leading unit parameter in a multi-parameter extern splits the call: unit -> int -> int = abs emits abs()(-5) #123 — a unit parameter now erases wherever it sits in an extern's parameter list, not only when it's the sole argument (breaking: a mid-list unit no longer arrives as Python None).

Test plan

• cargo test (418 passed)
• cargo clippy --all-targets (clean)
• After merge: tag v0.8.1, verify PyPI + VS Code Marketplace + Open VSX + JetBrains publish, set GitHub release notes from local/release-notes-0.8.1.md