added an interface for stack operations
added an interface for stack operations

schieb/angr,chubbymaggie/angr,chubbymaggie/simuvex,chubbymaggie/simuvex,tyb0807/angr,iamahuman/angr,tyb0807/angr,f-prettyland/angr,axt/angr,schieb/angr,f-prettyland/angr,axt/angr,axt/angr,iamahuman/angr,angr/angr,schieb/angr,chubbymaggie/angr,angr/angr,tyb0807/angr,f-prettyland/angr,chubbymaggie/simuvex,iamahuman/angr,angr/simuvex,chubbymaggie/angr,angr/angr,zhuyue1314/simuvex