More specific version of proptest, so it builds with Rust 1.65.

Proptest recently released 1.7, which bumps the minimum supported
Rust version to 1.66, and thus broke our CI. Since this is only a dev
dependency for running tests, there's little harm in locking it to
1.6.x, which allows CI to still run tests with Rust 1.65.

Author: cessen

Cargo.toml

@@ -28,7 +28,7 @@ str_indices = { version = "0.4", default-features = false }
 
 [dev-dependencies]
 rand = "0.8"
-proptest = "1.0"
+proptest = "~1.6"
 criterion = { version = "0.3", features = ["html_reports"] }
 unicode-segmentation = "1.3"
 fnv = "1"