== in tables should not raise KeyError

* With previous code, the compiler deduced that equalsImpl may raise
   a KeyError. While this could only actually happen in a nasty
   multi-threaded environment, I fixed the code so that it will never happen.

lib/pure/collections/tables.nim

@@ -304,8 +304,9 @@ template equalsImpl() =
     # to use the slow route here:
     for key, val in s:
       # prefix notation leads to automatic dereference in case of PTable
-      if not t.hasKey(key): return false
-      if t[key] != val: return false
+      try:
+        if t[key] != val: return false
+      except KeyError: return false
     return true
 
 proc `==`*[A, B](s, t: Table[A, B]): bool =