Replace compiletest suite by doc-tests with compile_fail

compiletest-rs use internal rustc APIs and is broken in today’s Nightly.
rustdoc however is maintained with rustc and so much less fragile.

tests/unit/deny_public_fields/lib.rs

@@ -0,0 +1,33 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+```compile_fail
+#[macro_use] extern crate deny_public_fields;
+
+#[derive(DenyPublicFields)]
+struct Foo {
+    pub v1: i32,
+    v2: i32
+}
+
+fn main() {}
+```
+*/
+pub fn deny_public_fields_failing() {}
+
+/**
+```
+#[macro_use] extern crate deny_public_fields;
+
+#[derive(DenyPublicFields)]
+struct Foo {
+    v1: i32,
+    v2: i32
+}
+
+fn main() {}
+```
+*/
+pub fn deny_public_fields_ok() {}