gnu: Add rust-base16ct-0.2.

* gnu/packages/crates-io.scm (rust-base16ct-0.2): New variable.

gnu/packages/crates-io.scm

@@ -5967,6 +5967,27 @@ systems.")
 API for Rust.")
     (license (list license:expat license:x11 license:asl2.0))))
 
+(define-public rust-base16ct-0.2
+  (package
+    (name "rust-base16ct")
+    (version "0.2.0")
+    (source (origin
+              (method url-fetch)
+              (uri (crate-uri "base16ct" version))
+              (file-name (string-append name "-" version ".tar.gz"))
+              (sha256
+               (base32
+                "1kylrjhdzk7qpknrvlphw8ywdnvvg39dizw9622w3wk5xba04zsc"))))
+    (build-system cargo-build-system)
+    (home-page "https://github.com/RustCrypto/formats/tree/master/base16ct")
+    (synopsis "Rust implementation of Base16 a.k.a hexadecimal")
+    (description
+     "This package provides a pure Rust implementation of Base16 a.k.a
+hexadecimal (RFC 4648) which avoids any usages of data-dependent branches/LUTs
+and thereby provides portable \"best effort\" constant-time operation and
+embedded-friendly no_std support.")
+    (license (list license:asl2.0 license:expat))))
+
 (define-public rust-base32-0.4
   (package
     (name "rust-base32")