Improve string serialization

[gmanvel]

This is a draft PR as it builds upon previous PR making use of Span based WriteBytes / WriteBytesRaw methods and using CoreBenchmarks project.

Changing string serialization to use Span based API. For strings that exceed 512 bytes (chosen arbitrarily) uses pooled byte arrays as a buffer otherwise uses stackalloc byte[byteCount].

Running String benchmark for various length string dotnet run -c Release --filter "*WriterBenchmarksString*" produces the following output

Method	StringLength (bytes)	Mean	Error	StdDev	Median	Ratio	RatioSD	Gen0	Allocated	Alloc Ratio
WriteString_Old	128	44.40 ns	0.875 ns	1.008 ns	44.26 ns	1.00	0.00	0.0363	152 B	1.00
WriteString_New	128	28.19 ns	0.475 ns	0.397 ns	28.19 ns	0.63	0.02	-	-	0.00
WriteString_Old	256	60.11 ns	1.504 ns	4.241 ns	58.39 ns	1.00	0.00	0.0669	280 B	1.00
WriteString_New	256	46.69 ns	0.827 ns	1.404 ns	46.22 ns	0.76	0.06	-	-	0.00
WriteString_Old	512	88.43 ns	1.697 ns	2.927 ns	87.36 ns	1.00	0.00	0.1281	536 B	1.00
WriteString_New	512	63.88 ns	1.314 ns	3.098 ns	63.05 ns	0.72	0.04	-	-	0.00
WriteString_Old	1024	160.01 ns	0.772 ns	0.645 ns	160.20 ns	1.00	0.00	0.2503	1048 B	1.00
WriteString_New	1024	84.57 ns	1.708 ns	1.426 ns	85.04 ns	0.53	0.01	-	-	0.00
WriteString_Old	2048	284.60 ns	5.521 ns	9.813 ns	280.61 ns	1.00	0.00	0.4950	2072 B	1.00
WriteString_New	2048	129.80 ns	2.553 ns	6.261 ns	128.34 ns	0.46	0.03	-	-	0.00
WriteString_Old	4096	519.27 ns	9.825 ns	10.090 ns	518.72 ns	1.00	0.00	0.9842	4120 B	1.00
WriteString_New	4096	217.34 ns	3.843 ns	6.096 ns	216.54 ns	0.42	0.02	-	-	0.00