Implement fmt::Debug for all structures in libstd.

Part of https://github.com/rust-lang/rust/issues/31869.

Also turn on the `missing_debug_implementations` lint at the crate
level.

src/libstd/env.rs

@@ -143,6 +143,13 @@ impl Iterator for Vars {
     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for Vars {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("Vars { .. }")
+    }
+}
+
 #[stable(feature = "env", since = "1.0.0")]
 impl Iterator for VarsOs {
     type Item = (OsString, OsString);
@@ -150,6 +157,13 @@ impl Iterator for VarsOs {
     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for VarsOs {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("VarsOs { .. }")
+    }
+}
+
 /// Fetches the environment variable `key` from the current process.
 ///
 /// The returned result is `Ok(s)` if the environment variable is present and is
@@ -364,6 +378,13 @@ impl<'a> Iterator for SplitPaths<'a> {
     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl<'a> fmt::Debug for SplitPaths<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("SplitPaths { .. }")
+    }
+}
+
 /// Error type returned from `std::env::join_paths` when paths fail to be
 /// joined.
 #[derive(Debug)]
@@ -640,6 +661,13 @@ impl DoubleEndedIterator for Args {
     }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for Args {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("Args { .. }")
+    }
+}
+
 #[stable(feature = "env", since = "1.0.0")]
 impl Iterator for ArgsOs {
     type Item = OsString;
@@ -657,6 +685,14 @@ impl ExactSizeIterator for ArgsOs {
 impl DoubleEndedIterator for ArgsOs {
     fn next_back(&mut self) -> Option<OsString> { self.inner.next_back() }
 }
+
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for ArgsOs {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("ArgsOs { .. }")
+    }
+}
+
 /// Constants associated with the current target
 #[stable(feature = "env", since = "1.0.0")]
 pub mod consts {