Add concrete IPv4 and IPv6 types

[glbrntt]

Motivation:

The ARecord and AAAARecord use the IPAddress enum as their address types. However, they should only use IPv4 and IPv6 addresses respectively.

Modifications:

• Add IPAddress.IPv4 and IPAddress.IPv6types and use them as the associated values in theIPAddress` enum
• Change the casing of the IPAddress cases from .IPv4 and .IPv6 to .ipv4 and .ipv6 as Swift case names are usually lower-camel case

Result:

• The types of IP address used by ARecord and AAAARecord are clearer.
• It's not possible to create an ARecord with an IPv6 address