constrain smallvec to a safe version range

see: https://rustsec.org/advisories/RUSTSEC-2021-0003.html

Cargo.toml

@@ -56,7 +56,7 @@ bytes = { version = "1", default-features = false }
 bytestring = { version = ">=1", optional = true }
 const_panic = { version = "0.2", optional = true }
 hashbrown = { version = ">=0.1.0", optional = true }
-smallvec = { version = ">=1", features = ["const_generics"], optional = true }
+smallvec = { version = ">=1.6.1", features = ["const_generics"], optional = true }
 thin-vec = { version = ">=0.2", optional = true }
 tinyvec = { version = ">=1", features = ["alloc"], optional = true }