post 2.0.0 fixes

[c-robinson]

Branch has a few things in it:

• attempt to get go.mod functional for v2 branch with the iana and iid submodules
• appease angry race detector (in linux only) on Net6.Enumerate() by cloning thread-local variables for IP and count
• add benchmarks and nilness tests for Net.Enumerate()
• appease angry syntax detector (in linux only) when net.go didn't have a newline after the final '}' in the file
• run golangci-lint (./golangci-lint run ./...) and then fix what it found
• swap out circleci/coveralls for a github actions, which run against linux, macos and windows for all explicitly accepted versions (note the race detector does not run under windows)