hide isa detection

[bitfaster]

Make a base class to hide the DetectIsa struct from normal use. Does not impact runtime performance:

Method	Runtime	Mean	Error	StdDev	Ratio	Allocated
ConcurrentDictionary	.NET 6.0	7.471 ns	0.0390 ns	0.0326 ns	1.00	-
ConcurrentLfuBackground	.NET 6.0	29.766 ns	0.5717 ns	1.0737 ns	3.95	-
ConcurrentLfuForeround	.NET 6.0	61.505 ns	0.1016 ns	0.0848 ns	8.23	-
ConcurrentLfuThreadPool	.NET 6.0	33.481 ns	0.5521 ns	0.5164 ns	4.49	-
ConcurrentLfuNull	.NET 6.0	27.202 ns	0.0374 ns	0.0332 ns	3.64	-
ConcurrentDictionary	.NET Framework 4.8	14.003 ns	0.1044 ns	0.0977 ns	1.00	-
ConcurrentLfuBackground	.NET Framework 4.8	73.827 ns	1.2531 ns	1.4917 ns	5.28	-
ConcurrentLfuForeround	.NET Framework 4.8	87.654 ns	0.2030 ns	0.1695 ns	6.25	-
ConcurrentLfuThreadPool	.NET Framework 4.8	60.264 ns	0.2999 ns	0.2659 ns	4.30	-
ConcurrentLfuNull	.NET Framework 4.8	33.919 ns	0.3026 ns	0.2683 ns	2.42	-