Update generator requirement from 0.7 to 0.8

Updates the requirements on [generator](https://github.com/Xudong-Huang/generator-rs) to permit the latest version.
- [Release notes](https://github.com/Xudong-Huang/generator-rs/releases)
- [Commits](Xudong-Huang/generator-rs@0.7.0...0.8.0)

---
updated-dependencies:
- dependency-name: generator
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Cargo.toml

@@ -26,7 +26,7 @@ bytes = { version = "1.2.1", optional = true }
 [dev-dependencies]                                    
 serde = { version = "1.0", features = ["derive"] }
 assert_matches = "1.5.0"
-generator = "0.7"
+generator = "0.8"
 itertools = "0.12.0"
 
 [package.metadata.docs.rs]