Motivation:

NIO's ByteBuffer is used as an implementation detail for parsing
responses in the DNSD implementation. However NIO is quite a heavy
dependency so we should avoid it if possible.

Modifications:

- Use `ArraySlice<UInt8>` instead of `ByteBuffer` and remove the
  dependency on SwiftNIO

Result:

No dependency on SwiftNIO

Motivation:
Having `platforms` in `Package.swift` might prevent others from adopting this library.

Modifications:
- Move platform availability to types
- Remove `platforms` in `Package.swift`
- macOS 10.15 seems to be sufficient, so lowering it from v13.0

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.IPv6` types and use them as the
  associated values in the `IPAddress` 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