gnu: Add rust-hex-0.4.

* gnu/packages/crates-io.scm (rust-hex-0.4): New variable.
(rust-hex-0.3): Inherit from rust-hex-0.4.

Signed-off-by: Efraim Flashner <efraim@flashner.co.il>

gnu/packages/crates-io.scm

@@ -4303,8 +4303,31 @@ (define-public rust-heapsize-plugin-0.1
 total runtime size of an object on the heap")
     (license license:mpl2.0)))
 
+(define-public rust-hex-0.4
+  (package
+    (name "rust-hex")
+    (version "0.4.0")
+    (source
+      (origin
+        (method url-fetch)
+        (uri (crate-uri "hex" version))
+        (file-name
+          (string-append name "-" version ".tar.gz"))
+        (sha256
+         (base32
+          "0glsfrx2pxfsf6ivxj7vfrvd7g78j4z47ssgm5idm8p376z3jfq2"))))
+    (build-system cargo-build-system)
+    (arguments '(#:skip-build? #t))
+    (home-page "https://github.com/KokaKiwi/rust-hex")
+    (synopsis "Encode and decode data to/from hexadecimals")
+    (description "This crate allows for encoding and decoding data into/from
+hexadecimal representation.")
+    (license (list license:asl2.0
+                   license:expat))))
+
 (define-public rust-hex-0.3
   (package
+    (inherit rust-hex-0.4)
     (name "rust-hex")
     (version "0.3.2")
     (source
@@ -4314,15 +4337,7 @@ (define-public rust-hex-0.3
         (file-name (string-append name "-" version ".crate"))
         (sha256
          (base32
-          "0xsdcjiik5j750j67zk42qdnmm4ahirk3gmkmcqgq7qls2jjcl40"))))
-    (build-system cargo-build-system)
-    (arguments '(#:skip-build? #t))
-    (home-page "https://github.com/KokaKiwi/rust-hex")
-    (synopsis "Encode and decode data to/from hexadecimals")
-    (description "This crate allows for encoding and decoding data into/from
-hexadecimal representation.")
-    (license (list license:asl2.0
-                   license:expat))))
+          "0xsdcjiik5j750j67zk42qdnmm4ahirk3gmkmcqgq7qls2jjcl40"))))))
 
 (define-public rust-hex-literal-0.2
   (package