Xo v0.7.0

First tagged release. Spec draft 0.7.

• Language: request scoped arenas by default, use go packages, two phase shutdown, Mutex.with_read, BytesBuilder, rune and integer conversions, native contracts and fault unwinding, habit diagnostics (XO0177), clock.sleep_or_cancel, cancellable stdin, tuple JSON, Duration.parse (decisions 0041 to 0088).
• Backends: Go (default), LLVM, native arm64, --no-gc, --hybrid.
• Toolchain: xo version.
• Planned before 1.0: modules and workspaces (0086), std/time, native TLS, and decisions on the Appendix B questions.