add windows exception handling in C (#175)

lib/runtime-core/src/sys/windows/memory.rs

@@ -150,7 +150,8 @@ impl Drop for Memory {
     fn drop(&mut self) {
         if !self.ptr.is_null() {
             let success = unsafe { VirtualFree(self.ptr as _, self.size, MEM_DECOMMIT) };
-            assert_eq!(success, 0, "failed to unmap memory: {}", errno::errno());
+            // If the function succeeds, the return value is nonzero.
+            assert_eq!(success, 1, "failed to unmap memory: {}", errno::errno());
         }
     }
 }

lib/runtime-core/src/vm.rs

@@ -169,6 +169,7 @@ enum InnerFunc {}
 /// Used to provide type safety (ish) for passing around function pointers.
 /// The typesystem ensures this cannot be dereferenced since an
 /// empty enum cannot actually exist.
+#[repr(C)]
 pub struct Func(InnerFunc);
 
 /// An imported function, which contains the vmctx that owns this function.