The same piece of GossipData can be shared and subject to different
Merge operations. We either need to copy the data when we share it, or
create copies on modification, i.e. copy-on-write.
We choose the latter, since it is more efficient and there is just one
mutating operation.
Fixes#1697.
This is mostly a matter of moving some files, changing package names,
changing some imports, and changing some type references.
We duplicate the odd constant & global var, and the intmac/macint
functions, since neither 'router' or 'mesh' nor, really,' common' are
a natural home for them.
Matthias Radestock