Version 0.7.0

This release changes the structure of the Error types.

To upgrade, you should:

• Migrate uses of netaddr2::netaddr_error::NetAddrError to netaddr2::error::Error (much cleaner!)

• Consider adopting the new netaddr2::error::Result type, which, like the std::io::Error type, encapsulates your desired type with a Result whose variant is a netaddr2::error::Error.