SimEngine: fix the issue that callbacks cannot be pickled by CPython.
SimEngine: fix the issue that callbacks cannot be pickled by CPython.

angr/angr,chubbymaggie/angr,axt/angr,iamahuman/angr,f-prettyland/angr,schieb/angr,f-prettyland/angr,iamahuman/angr,axt/angr,f-prettyland/angr,tyb0807/angr,tyb0807/angr,axt/angr,chubbymaggie/angr,iamahuman/angr,chubbymaggie/angr,tyb0807/angr,schieb/angr,angr/angr,angr/angr,schieb/angr