nested struct assignment improvement

[justincormack]

At the moment when we have nested structs eg ifaliasreq, sockaddr_in we are doing a mktype in the newindex on assigning, which works but creates a temporary variable which is then copied. We should be able to assign directly by having a helper function that works like newfn. It needs to memset to zero (assignment not update) then assign each table element if passed a table, otherwise just assign as usual.