Improved performance of Utf16ValueStringBuilder.AppendFormat on Slow Span Environment

[udaken]

For environments like Mono and .NET Framework, I reduced the Span used in Utaf16ValueStringBuilder.AppendFormat.

Benchmark on .NET Framework

$ dotnet run -c Release --framework net47
Available Benchmarks:
  #0 AppendPerformance
  #1 FormatBenchmark
...
1

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19043
AMD Ryzen 5 5600X, 1 CPU, 12 logical and 6 physical cores
.NET Core SDK=6.0.100-preview.4.21255.9
  [Host]   : .NET Core 5.0.6 (CoreCLR 5.0.621.22011, CoreFX 5.0.621.22011), X64 RyuJIT  [AttachedDebugger]
  ShortRun : .NET Framework 4.8 (4.8.4341.0), X64 RyuJIT

Job=ShortRun  Runtime=.NET 4.7  IterationCount=1
LaunchCount=1  WarmupCount=1

Before

Method	Mean	Error	Ratio	Gen 0	Gen 1	Gen 2	Allocated
StringFormat	155.02 ns	NA	1.00	0.1013	-	-	169 B
ZStringFormat	149.62 ns	NA	0.97	0.0336	-	-	56 B
ZStringPreparedFormat	122.82 ns	NA	0.79	0.0336	-	-	56 B
StringFormatterFormat	81.76 ns	NA	0.53	0.0337	-	-	56 B

After

Method	Mean	Error	Ratio	Gen 0	Gen 1	Gen 2	Allocated
StringFormat	153.41 ns	NA	1.00	0.1013	-	-	169 B
ZStringFormat	133.61 ns	NA	0.87	0.0336	-	-	56 B
ZStringPreparedFormat	122.32 ns	NA	0.80	0.0336	-	-	56 B
StringFormatterFormat	81.04 ns	NA	0.53	0.0337	-	-	56 B

[github-actions]

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[neuecc]

sorry for the delaying review.
I'll check soon.

[neuecc]

sorry for the delay to merge, thanks!