Fix lifting an own handle from a borrow handle

This fixes what I believe is a mistake in the `CanonicalABI.md`
explainer currently where a lifting an owned resource handle from the
table can succeed if the entry in the table was actually a borrowed
handle. This inserts a check to ensure that the handle is an `own`
handle before completing the lifting operation.

Author: alexcrichton

design/mvp/CanonicalABI.md

@@ -641,6 +641,7 @@ component.
 ```python
 def lift_own(cx, i, t):
   h = cx.inst.handles.remove(t.rt, i)
+  trap_if(not h.own)
   return h.rep
 ```
 The abstract lifted value for handle types is currently just the internal