Auto merge of #254 - sibsibsib:master, r=jdm

to_i32 shortcut for CFNumber

kCFNumberIntType is needed in some iokit APIs.

Please let me know if you think anything is missing.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/core-foundation-rs/254)
<!-- Reviewable:end -->

core-foundation/src/number.rs

@@ -25,6 +25,16 @@ impl_CFTypeDescription!(CFNumber);
 impl_CFComparison!(CFNumber, CFNumberCompare);
 
 impl CFNumber {
+
+    #[inline]
+    pub fn to_i32(&self) -> Option<i32> {
+        unsafe {
+            let mut value: i32 = 0;
+            let ok = CFNumberGetValue(self.0, kCFNumberSInt32Type, mem::transmute(&mut value));
+            if ok { Some(value) } else { None }
+        }
+    }
+
     #[inline]
     pub fn to_i64(&self) -> Option<i64> {
         unsafe {