Avoid serialisation of references nightmare

Without this, large footpath networks turn into enormous files that take
forever to save. With it, they're small and instant.

footpath.lua

@@ -8,9 +8,22 @@ local routecache = {}
 --- Save the current footpath network status.
 people.footpath_save = function()
     if not pathnodes then return end
+
+    -- For the purposes of serialisation, we want a copy of the nodes with
+    -- neighbours referenced by node name, not a reference to the actual
+    -- node, otherwise it will serialise horribly!
+    local lnodes = {}
+    for n, nn in pairs(pathnodes) do
+        local nei = {}
+        for _, ne in pairs(nn.neighbours) do
+            table.insert(nei, ne.name)
+        end
+        lnodes[n] = {name=n, pos=nn.pos, neighbours=nei}
+    end
+
     local f = io.open(minetest.get_worldpath().."/people_footpaths", "w+")
     if f then
-        f:write(minetest.serialize(pathnodes))
+        f:write(minetest.serialize(lnodes))
         f:close()
     end
 end
@@ -21,6 +34,16 @@ people.footpath_load = function()
     if f then
         pathnodes = minetest.deserialize(f:read("*all"))
         f:close()
+
+        -- Now we need to undo the dereferencing we did when we saved it...
+        for _, nn in pairs(pathnodes) do
+            local nei = {}
+            for _, ne in pairs(nn.neighbours) do
+                table.insert(nei, pathnodes[ne])
+            end
+            nn.neighbours = nei
+        end
+
     end
 end