Introduce IPropertyDescriptor and specialized descriptors

[lahma]

This PR changes PropertyDescriptor usage to happen through IPropertyDescriptor interface. This allows specialized descritors that don't hold other instance fields other than the actual JsValue. This benefits especially arrays where descriptor stays quite constant. Array heave operators seem to get around 10% allocation reduction.

ObjectInstance converts to more memory hungry full PropertyDescriptor when changes are requested. I've kept new specialized versions as internal. This change affects memory usage and should not have much positive or negative impact on speed. Virtual dispatch is already happening with PropertyDescriptor.Value as it's virtual.

Before

Method	N	Mean	Error	StdDev	Gen 0	Allocated
Slice	100	877.4 us	7.603 us	7.112 us	239.2578	982.81 KB
Concat	100	991.0 us	1.550 us	1.210 us	257.8125	1062.5 KB
Unshift	100	36,026.6 us	182.054 us	152.023 us	7000.0000	28785.16 KB
Push	100	23,722.6 us	439.804 us	411.393 us	1625.0000	6762.5 KB
Index	100	25,124.1 us	59.588 us	49.759 us	1531.2500	6333.59 KB
Map	100	4,644.9 us	15.578 us	13.810 us	1804.6875	7420.31 KB
Apply	100	1,040.4 us	5.970 us	5.584 us	267.5781	1098.44 KB
JsonStringifyParse	100	6,641.6 us	26.941 us	25.200 us	1476.5625	6068.75 KB

Method	N	Mean	Error	StdDev	Gen 0	Gen 1	Allocated
UncacheableExpressionsBenchmark	500	572.1 ms	82.07 ms	4.637 ms	114750.0000	33562.5000	494.73 MB

After

Method	N	Mean	Error	StdDev	Gen 0	Allocated
Slice	100	839.7 us	6.128 us	5.432 us	154.2969	635.94 KB
Concat	100	947.8 us	11.131 us	10.412 us	171.8750	707.81 KB
Unshift	100	37,094.3 us	278.159 us	260.190 us	3750.0000	15492.19 KB
Push	100	25,588.8 us	30.847 us	27.345 us	1468.7500	6125.78 KB
Index	100	26,047.0 us	102.136 us	90.541 us	1375.0000	5696.88 KB
Map	100	4,650.9 us	9.938 us	9.296 us	1718.7500	7071.88 KB
Apply	100	1,038.3 us	3.905 us	3.653 us	187.5000	774.22 KB
JsonStringifyParse	100	6,671.2 us	6.482 us	6.063 us	1390.6250	5710.16 KB

Method	N	Mean	Error	StdDev	Gen 0	Gen 1	Allocated
UncacheableExpressionsBenchmark	500	592.7 ms	22.18 ms	1.253 ms	100104.1667	29541.6667	446.99 MB

[sebastienros]

Looks good.

[lahma]

Thanks. End user API changes with IPropertyDescriptor being returned/taken as parameter (PropertyDescriptor works as before for defining things). Also the methods IsDataDescriptor() etc moved to be extension methods to help interface-only implementations.

Feel free to merge if/when you find suitable, all tweaks naturally welcome.

[sebastienros]

I was just wondering if it would be better to introduce [Flags]. I know I have tried by the past, but I think it was more pain than this. Unless there are helper methods.

[lahma]

Yes, that's an alternative. Without interface it would marginally faster. Of course the flags need to be stored as field but getters could do bit checks. The extra Get and Set would be hard to model without properties though.

But I'm in no hurry with this one. The last merge brought the speed improvements already, this is tweaking and balancing now.