Hide "platform-intrinsic" from stable Rust

The Rust compiler validates the extern ABI while parsing the "extern"
keyword, so normal conditional compilation (`#[cfg(...)]`) isn't enough
to hide the ABI from Rust versions which don't know it.

I tried hiding the extern ABI using a macro, but the contents of an
"extern" block aren't a valid `item`, and I couldn't find any other
working way to pass the function declarations to the macro.

The solution which worked in the end was to use `include!`. This
prevents the compiler from even trying to parse the "extern" block
unless the nightly-only cargo feature "simd" is enabled.

src/lib.rs

@@ -39,7 +39,10 @@ mod as_bytes;
 mod bytes;
 
 mod simdty;
-#[cfg(feature = "simd")] mod simdint;
+#[allow(dead_code)] mod simdint {
+    #[cfg(feature = "simd")]
+    include!("simdint.rs");
+}
 mod simd;
 
 #[macro_use]

src/simdint.rs

@@ -24,8 +24,6 @@
 // IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-#![allow(dead_code)]
-
 use simdty;
 
 extern "platform-intrinsic" {