Implement IUtf8SpanFormattable on IPAddress and IPNetwork

[stephentoub]

Implements IUtf8SpanFormattable explicitly on both IPAddress and IPNetwork. For IPNetwork, we just use Utf8.TryWrite just as the existing ISpanFormattable uses MemoryExtensions.TryWrite. For IPAddress, the existing formatting code is made to work generically for either byte or char.

In the process, I removed the unsafe pointer-based code from the formatting logic while also making it faster.

Method	Toolchain	Mean	Ratio
TryFormatIPv4	\main\corerun.exe	19.20 ns	1.00
TryFormatIPv4	\pr\corerun.exe	15.22 ns	0.79
TryFormatIPv6	\main\corerun.exe	81.15 ns	1.00
TryFormatIPv6	\pr\corerun.exe	50.50 ns	0.62

private char[] _buffer = new char[100];
private IPAddress _ipv4 = IPAddress.Parse("20.49.104.40");
private IPAddress _ipv6 = IPAddress.Parse("2600:1400:000d:059e:0000:0000:0000:356e");

[Benchmark]
public bool TryFormatIPv4() => _ipv4.TryFormat(_buffer, out _);

[Benchmark]
public bool TryFormatIPv6() => _ipv6.TryFormat(_buffer, out _);

This also fixes #84482.

Contributes to #81500

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Tagging subscribers to this area: @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Implements IUtf8SpanFormattable explicitly on both IPAddress and IPNetwork. For IPNetwork, we just use Utf8.TryWrite just as the existing ISpanFormattable uses MemoryExtensions.TryWrite. For IPAddress, the existing formatting code is made to work generically for either byte or char.

In the process, I removed the unsafe pointer-based code from the formatting logic while also making it faster.

Method	Toolchain	Mean	Ratio
TryFormatIPv4	\main\corerun.exe	19.20 ns	1.00
TryFormatIPv4	\pr\corerun.exe	15.22 ns	0.79
TryFormatIPv6	\main\corerun.exe	81.15 ns	1.00
TryFormatIPv6	\pr\corerun.exe	50.50 ns	0.62

This also fixes #84482.

Contributes to #81500

Author:	stephentoub
Assignees:	-
Labels:	area-System.Net
Milestone:	8.0.0

[wfurt]

LGTM