Changed Ipv4-compatible to Ipv4-mapped addresses

[rvbcldud]

Background

This crate states that it supports the use of Ipv4 CIDR notation when defining a range for the IP generator to use. However, during my tests using it I did not find that to be the case. Rather, I got the following error:

Error: InvalidIpNetwork("[BUG] failed to parse the generated IP address `a83:6304` as IPv4")

Furthermore, the test currently fail in lib.rs

Changes

After doing some debugging within the crate itself, I found that when converting from Ipv4 to Ipv6, Ipv4-compatible Ipv6 addresses were used. This type of address "was defined to assist in the IPv6 transition" and is now deprecated as of RFC 4291. Instead of prepending a ::, in the case of the Ipv4-compatible address, ip() should prepend ::fff: (and likewise strip).

Conclusion

This fixes the issues I encountered when trying to generate Ipv4 addresses using this crate and restores the tests in lib.rs to working order.

[rushmorem]

Thank you!