Use System.Memory with netstandard2.0 target

[lahma]

This simplifies code and I think such dependency is quite natural for parser library. Only a suggestion though. Also upgraded libs and run benchmarks to see where Parlot's at.

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.22000
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK=6.0.101
  [Host]   : .NET 6.0.1 (6.0.121.56705), X64 RyuJIT
  ShortRun : .NET 6.0.1 (6.0.121.56705), X64 RyuJIT

Job=ShortRun  IterationCount=3  LaunchCount=1  
WarmupCount=3  

Method	Mean	Error	StdDev	Ratio	RatioSD	Gen 0	Gen 1	Allocated
BigJson_Parlot	132.35 μs	22.934 μs	1.257 μs	1.00	0.00	6.1035	1.7090	102 KB
BigJson_ParlotCompiled	113.47 μs	4.514 μs	0.247 μs	0.86	0.01	6.2256	2.0752	102 KB
BigJson_Pidgin	224.43 μs	12.236 μs	0.671 μs	1.70	0.01	6.1035	1.4648	102 KB
BigJson_Sprache	1,511.30 μs	79.740 μs	4.371 μs	11.42	0.14	322.2656	107.4219	5,274 KB
BigJson_Superpower	1,024.03 μs	76.137 μs	4.173 μs	7.74	0.05	54.6875	15.6250	913 KB
LongJson_Parlot	105.03 μs	6.730 μs	0.369 μs	1.00	0.00	6.3477	1.5869	104 KB
LongJson_ParlotCompiled	88.67 μs	5.642 μs	0.309 μs	0.84	0.00	6.3477	1.5869	104 KB
LongJson_Pidgin	188.68 μs	22.686 μs	1.244 μs	1.80	0.01	6.3477	1.7090	104 KB
LongJson_Sprache	1,304.92 μs	217.296 μs	11.911 μs	12.42	0.15	259.7656	70.3125	4,245 KB
LongJson_Superpower	845.62 μs	12.114 μs	0.664 μs	8.05	0.03	42.9688	10.7422	707 KB
DeepJson_Parlot	86.60 μs	4.453 μs	0.244 μs	1.00	0.00	5.0049	0.8545	82 KB
DeepJson_ParlotCompiled	63.64 μs	0.871 μs	0.048 μs	0.73	0.00	5.0049	1.0986	82 KB
DeepJson_Pidgin	269.30 μs	23.575 μs	1.292 μs	3.11	0.01	9.2773	2.9297	153 KB
DeepJson_Sprache	1,031.54 μs	134.960 μs	7.398 μs	11.91	0.05	175.7813	64.4531	2,898 KB
WideJson_Parlot	78.25 μs	17.981 μs	0.986 μs	1.00	0.00	2.9297	0.4883	49 KB
WideJson_ParlotCompiled	74.11 μs	9.759 μs	0.535 μs	0.95	0.02	2.9297	0.6104	49 KB
WideJson_Pidgin	125.94 μs	19.652 μs	1.077 μs	1.61	0.03	2.9297	0.4883	48 KB
WideJson_Sprache	742.31 μs	51.826 μs	2.841 μs	9.49	0.09	168.9453	41.9922	2,761 KB
WideJson_Superpower	518.36 μs	31.894 μs	1.748 μs	6.62	0.06	27.3438	4.8828	460 KB