symbol files: do not regenerate method dispatchers for now

compiler/cgen.nim

@@ -1443,6 +1443,10 @@ proc myClose(graph: ModuleGraph; b: PPassContext, n: PNode): PNode =
   result = n
   if b == nil or passes.skipCodegen(n): return
   var m = BModule(b)
+  # if the module is cached, we don't regenerate the main proc
+  # nor the dispatchers? But if the dispatchers changed?
+  # XXX emit the dispatchers into its own .c file?
+  if b.rd != nil: return
   if n != nil:
     m.initProc.options = initProcOptions(m)
     genStmts(m.initProc, n)