gnu: Add rust-arbitrary-0.4.

* gnu/packages/crates-io.scm (rust-arbitrary-0.4): New variable.
(rust-arbitrary-0.2): Inherit from above.

gnu/packages/crates-io.scm

@@ -1903,8 +1903,38 @@ on every platform.")
          "153awzwywmb61xg857b80l63b1x6hifx2pha7lxf6fck9qxwraq8"))))
     (arguments '())))
 
+(define-public rust-arbitrary-0.4
+  (package
+    (name "rust-arbitrary")
+    (version "0.4.7")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (crate-uri "arbitrary" version))
+       (file-name (string-append name "-" version ".tar.gz"))
+       (sha256
+        (base32 "0sa55cynafwzvlhyhfpm3vmi2fydj3ipdj5yfbaif7l56cixfmfv"))))
+    (build-system cargo-build-system)
+    (arguments
+     `(#:skip-build? #t
+       #:cargo-inputs
+       (("rust-derive-arbitrary" ,rust-derive-arbitrary-0.4))))
+    (home-page "https://github.com/nagisa/rust_arbitrary")
+    (synopsis "Trait for generating structured data from unstructured data")
+    (description
+     "The @code{Arbitrary} crate lets you construct arbitrary instance of
+a type.
+
+This crate is primarily intended to be combined with a fuzzer like
+@code{libFuzzer} and @code{cargo-fuzz} or AFL, and to help you turn the raw,
+untyped byte buffers that they produce into well-typed, valid, structured
+values.  This allows you to combine structure-aware test case generation with
+coverage-guided, mutation-based fuzzers.")
+    (license (list license:expat license:asl2.0))))
+
 (define-public rust-arbitrary-0.2
   (package
+    (inherit rust-arbitrary-0.4)
     (name "rust-arbitrary")
     (version "0.2.0")
     (source
@@ -1915,13 +1945,7 @@ on every platform.")
           (string-append name "-" version ".tar.gz"))
         (sha256
           (base32
-            "1i3fhcdyjq4isn22xx2svmpfr5hwyzi0wavbm07fs8i2dv5pdkv4"))))
+            "1i3fhcdyjq4isn22xx2svmpfr5hwyzi0wavbm07fs8i2dv5pdkv4"))))))
-    (build-system cargo-build-system)
-    (home-page "https://github.com/nagisa/rust_arbitrary/")
-    (synopsis "Trait for generating structured data from unstructured data")
-    (description
-     "The trait for generating structured data from unstructured data.")
-    (license (list license:expat license:asl2.0))))
 
 (define-public rust-arc-swap-1
   (package