Implement IUtf8SpanFormattable on Complex

[stephentoub]

Depends on #84587
Contributes to #81500

Method	Toolchain	Mean	Error	StdDev	Ratio	RatioSD	Code Size	Allocated	Alloc Ratio
ToStringDefault	\main\corerun.exe	220.7 ns	4.06 ns	3.60 ns	1.00	0.00	439 B	48 B	1.00
ToStringDefault	\pr\corerun.exe	203.7 ns	4.08 ns	4.00 ns	0.93	0.03	525 B	48 B	1.00
ToStringArgs	\main\corerun.exe	243.2 ns	1.31 ns	1.10 ns	1.00	0.00	2,686 B	112 B	1.00
ToStringArgs	\pr\corerun.exe	195.0 ns	0.78 ns	0.65 ns	0.80	0.00	543 B	48 B	0.43
TryFormat	\main\corerun.exe	171.0 ns	1.97 ns	1.65 ns	1.00	0.00	642 B	-	NA
TryFormat	\pr\corerun.exe	170.6 ns	0.30 ns	0.24 ns	1.00	0.01	589 B	-	NA

private Complex _c = new Complex(12.1, 15.4);
private char[] _chars = new char[100];

[Benchmark]
public string ToStringDefault() => _c.ToString();

[Benchmark]
public string ToStringArgs() => _c.ToString("G", CultureInfo.InvariantCulture);

[Benchmark]
public bool TryFormat() => _c.TryFormat(_chars, out _, default, CultureInfo.InvariantCulture);

[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.

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Issue Details

---

Depends on #84587

Method	Toolchain	Mean	Error	StdDev	Ratio	RatioSD	Code Size	Allocated	Alloc Ratio
ToStringDefault	\main\corerun.exe	220.7 ns	4.06 ns	3.60 ns	1.00	0.00	439 B	48 B	1.00
ToStringDefault	\pr\corerun.exe	203.7 ns	4.08 ns	4.00 ns	0.93	0.03	525 B	48 B	1.00
ToStringArgs	\main\corerun.exe	243.2 ns	1.31 ns	1.10 ns	1.00	0.00	2,686 B	112 B	1.00
ToStringArgs	\pr\corerun.exe	195.0 ns	0.78 ns	0.65 ns	0.80	0.00	543 B	48 B	0.43
TryFormat	\main\corerun.exe	171.0 ns	1.97 ns	1.65 ns	1.00	0.00	642 B	-	NA
TryFormat	\pr\corerun.exe	170.6 ns	0.30 ns	0.24 ns	1.00	0.01	589 B	-	NA

private Complex _c = new Complex(12.1, 15.4);
private char[] _chars = new char[100];

[Benchmark]
public string ToStringDefault() => _c.ToString();

[Benchmark]
public string ToStringArgs() => _c.ToString("G", CultureInfo.InvariantCulture);

[Benchmark]
public bool TryFormat() => _c.TryFormat(_chars, out _, default, CultureInfo.InvariantCulture);

Author:	stephentoub
Assignees:	stephentoub
Labels:	area-System.Numerics, new-api-needs-documentation
Milestone:	-

[stephentoub]

This was rebased on the corelib additions and is ready for review.