AssertUnwindSafe and Arc/Rc wrappers

src/context.rs

+use std::rc::Rc;
+use std::sync::Arc;
 use super::*;
 use std::ptr;
 use std::mem;
@@ -19,6 +21,9 @@ pub struct GlobalContext {
 impl UnwindSafe for GlobalContext {}
 impl RefUnwindSafe for GlobalContext {}
 
+impl UnwindSafe for ThreadContext {}
+impl RefUnwindSafe for ThreadContext {}
+
 #[doc(hidden)]
 pub trait Context {
     fn as_ptr(&self) -> ffi::Context;
@@ -50,6 +55,20 @@ impl<'a> Context for &'a ThreadContext {
     }
 }
 
+impl<'a> Context for Arc<ThreadContext> {
+    #[inline]
+    fn as_ptr(&self) -> ffi::Context {
+        self.handle
+    }
+}
+
+impl<'a> Context for Rc<ThreadContext> {
+    #[inline]
+    fn as_ptr(&self) -> ffi::Context {
+        self.handle
+    }
+}
+
 impl Context for ThreadContext {
     #[inline]
     fn as_ptr(&self) -> ffi::Context {