Add IPToBinarySlice function (#17)

This commmit adds IPToBinarySlice. Given a net.IP, it will return a
[]byte whose values are the binary representation of the IP.

0 is a valid mask length

Nets betweening and tests

 - Fix NewNetBetween to be more predictable in cases where the resu;t is a
   single-address network

 - add AllNetsBetween which returns all the netblocks between two supplied
   addresses

 - improve the test cases to eliminate duplicatation and provide equivalent
   coverage for v4 and v6

post 2.0.0 fixes (#16)

* ...aaaaaaand update doc references as well

* it is not obvious how go.mod should work for submods

* v2 before submodule name in replace

* fix docs for new go.mod approach

* add benchmark for enumerate

* make sure all the addresses are actually present

* add linefeed at end to avoid go test error

* race detector didnt like var re-use

* fixes for 'golangci-lint run'

* swap over from circleci/coveralls to actions

* update readme's for new test badges

* explicitly pick a golangci action version

update documentation for v2 in import path

remove indirect

Fix edge case in DecrementIP6WithinHostmask

 - Fix a problem where DecrementIP6WithinHostmask could panic if
   the hostmask had an underflow

 - Added a benchmark for subnetting IPv6

avoid index out of bounds in IP4ToARPA function (#14)

tests for CopyIP() and RandomIP()

fix: NextNet with mask length lower than current one (#13)

Signed-off-by: Luis Davim <[email protected]>

Signed-off-by: Luis Davim <[email protected]>