CFW breaks duplicator due to cycles

[Python1320]

Notes:

• Version: latest git
• Unverified in singleplayer yet without other addons, sorry :(

Steps to reproduce:

1. Spawn two props
2. Rope them together
3. in console try executing dupe_save
4. Observe error in console:

      gamemodes/sandbox/entities/weapons/gmod_tool/stools/duplicator/transport.lua:30: 
               attempt to serialize structure with cyclic reference

The issue is due to the entity table having cycles, namely _contraption's CostObj:

I don't know the codebase enough but likely it seems to be related to

CFW/lua/cfw/classes/contraption_sv.lua

Line 31 in 5bb6f9f

-- This function has caused headaches in other codebases (wiremod's cam controllers for example) and should've always been namedspaced...

Or rather CFW_GetContraption should do something like:

local cfw_contraptions = {}
...
    function ENT:CFW_GetContraption()
        return cfw_contraptions[self]
    end
...

    function CLASS:Add(ent)
        cfw_contraptions[ent] = self
    end

(and same for links, etc)