add windows exception handling in C (#175)

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.