pin certain crates to specific versions for msrv

Author: hellow554

Cargo.toml

@@ -43,6 +43,9 @@ approx = "0.5"
 quickcheck = "1.0"
 serde_json = "1.0"
 static_assertions = "1.1"
+# TODO: once the msrv is bumped, we can relax these constraints
+regex = "=1.9"
+memchr = "=2.6.2"
 
 [features]
 default = ["autoconvert", "f32", "f64", "si", "std"]