Work around nightly rustc breaking type inference

It seems there was a type inference regression in Nightly rustc, where returning
an Ok literal at the end of a function whose type signature returns a Result is
failing. I also hit this on some other projects.

Author: fitzgen

src/cfi.rs

@@ -304,7 +304,7 @@ pub trait UnwindSection<'input, Endian>
     /// # let do_stuff_with = |_| unimplemented!();
     /// do_stuff_with(unwind_info);
     /// # let _ = ctx;
-    /// # Ok(())
+    /// # unreachable!()
     /// # }
     /// ```
     fn unwind_info_for_address<'bases>
@@ -571,7 +571,7 @@ impl BaseAddresses {
 /// while let Some(entry) = try!(entries.next()) {
 ///     do_stuff_with(entry)
 /// }
-/// # Ok(())
+/// # unreachable!()
 /// # }
 /// ```
 #[derive(Clone, Debug)]
@@ -1263,7 +1263,7 @@ impl<'input, Endian, Section> FrameDescriptionEntry<'input, Endian, Section>
 /// // programs.
 /// let ctx = ctx.reset();
 /// # let _ = ctx;
-/// # Ok(())
+/// # unreachable!()
 /// # }
 /// ```
 ///

src/lib.rs

@@ -48,7 +48,7 @@
 //!         }
 //!     }
 //! }
-//! # Ok(())
+//! # unreachable!()
 //! # }
 //! ```
 //!

src/unit.rs

@@ -2264,7 +2264,7 @@ impl<'input, 'abbrev, 'unit, Endian> EntriesCursor<'input, 'abbrev, 'unit, Endia
 ///
 /// let mut tree = try!(unit.entries_tree(&abbrevs, None));
 /// try!(process_tree(tree.iter()));
-/// # Ok(())
+/// # unreachable!()
 /// # }
 ///
 /// fn process_tree<E>(mut iter: gimli::EntriesTreeIter<E>) -> gimli::Result<()>