Import NAT code into dataplane repo

[qmonnet]

Our code for NAT is currently in a separate, private repository.

There's no good reason to separate it, it was simply to make it easier to iterate for the early stage of the code.

There's no good reason to keep it private, although we must make sure that headers with licensing and copyright attribution are present before moving the files.

We need to clean up (?) the other repo and import all code into dataplane at some point.

[qmonnet]

Now done. I just archived the tmp-nat private repository.