feat: Short-hand Assert::code

Part of #2

src/assert.rs

@@ -154,7 +154,15 @@ impl Assert {
     ///     .assert()
     ///     .code(predicates::ord::eq(42));
     /// ```
-    pub fn code(self, pred: &predicates::Predicate<i32>) -> Self {
+    pub fn code<I, P>(self, pred: I) -> Self
+    where
+        I: IntoCodePredicate<P>,
+        P: predicates::Predicate<i32>,
+    {
+        self.code_impl(&pred.into_code())
+    }
+
+    fn code_impl(self, pred: &predicates::Predicate<i32>) -> Self {
         let actual_code = self.output
             .status
             .code()

tests/assert.rs

@@ -0,0 +1,22 @@
+extern crate assert_cmd;
+extern crate predicates;
+
+use std::process::Command;
+
+use assert_cmd::prelude::*;
+use predicates::prelude::*;
+
+#[test]
+fn code_example() {
+    Command::main_binary()
+        .unwrap()
+        .env("exit", "42")
+        .assert()
+        .code(42);
+    // which is equivalent to
+    Command::main_binary()
+        .unwrap()
+        .env("exit", "42")
+        .assert()
+        .code(predicate::eq(42));
+}