Optimize PropertyDescriptor with flags

[lahma]

We get more performance with no need to do interface dispatch for common property descriptor properties. Some memory penalty but the speed increase should be worth it - can check later on can we reduce memory usage without sacrificing speed.

• optimize methods and make tweaks for better inlining
• use flags enum for PropertyDescriptor
• remove Get and Set backing fields from PropertyDescriptor, separate GetSetPropertyDescriptor
• remove IPropertyDescriptor now that we have fast and memory efficient PropertyDescriptor

Esprima.Benchmark.DromaeoBenchmark

Diff	Method	FileName	Mean	Gen 0	Gen 1	Gen 2	Allocated
Old	Run	dromaeo-3d-cube	103.35 ms	1312.5000	250.0000	-	7107.59 KB
New			95.37 ms (-8%)	1375.0000 (+5%)	312.5000 (+25%)	-	7349.61 KB (+3%)
Old	Run	dromaeo-core-eval	24.70 ms	62.5000	-	-	298.95 KB
New			22.53 ms (-9%)	62.5000 (0%)	-	-	299.09 KB (0%)
Old	Run	dromaeo-object-array	259.58 ms	39500.0000	2000.0000	1000.0000	166027.24 KB
New			216.87 ms (-16%)	43250.0000 (+9%)	2000.0000 (0%)	1000.0000 (0%)	181394.68 KB (+9%)
Old	Run	dromaeo-object-regexp	1,142.73 ms	57750.0000	34750.0000	22125.0000	433170.74 KB
New			1,185.22 ms (+4%)	59437.5000 (+3%)	33000.0000 (-5%)	22312.5000 (+1%)	451764.26 KB (+4%)
Old	Run	dromaeo-object-string	1,359.54 ms	220812.5000	178437.5000	175812.5000	1392803.07 KB
New			1,312.64 ms (-3%)	223250.0000 (+1%)	181000.0000 (+1%)	177937.5000 (+1%)	1395016.54 KB (0%)
Old	Run	dromaeo-string-base64	234.61 ms	6187.5000	187.5000	-	26458.12 KB
New			221.46 ms (-6%)	6437.5000 (+4%)	375.0000 (+100%)	-	27675.41 KB (+5%)

Jint.Benchmark.ArrayBenchmark

Diff	Method	N	Mean	Gen 0	Allocated
Old	Slice	100	671.5 us	145.5078	598.44 KB
New			585.8 us (-13%)	161.1328 (+11%)	662.5 KB (+11%)
Old	Concat	100	797.6 us	162.1094	666.41 KB
New			683.0 us (-14%)	177.7344 (+10%)	728.91 KB (+9%)
Old	Unshift	100	30,480.2 us	3031.2500	12525.78 KB
New			24,312.1 us (-20%)	3625.0000 (+20%)	14931.25 KB (+19%)
Old	Push	100	17,942.4 us	593.7500	2518.75 KB
New			15,271.4 us (-15%)	578.1250 (-3%)	2392.97 KB (-5%)
Old	Index	100	16,595.0 us	343.7500	1515.63 KB
New			14,445.7 us (-13%)	390.6250 (+14%)	1642.97 KB (+8%)
Old	Map	100	4,150.7 us	796.8750	3286.72 KB
New			4,070.1 us (-2%)	812.5000 (+2%)	3353.91 KB (+2%)
Old	Apply	100	933.3 us	176.7578	727.34 KB
New			780.7 us (-16%)	192.3828 (+9%)	789.06 KB (+8%)
Old	JsonStringifyParse	100	5,289.6 us	1242.1875	5111.72 KB
New			4,996.3 us (-6%)	1257.8125 (+1%)	5175 KB (+1%)

Jint.Benchmark.ArrayStressBenchmark

Diff	Method	N	Mean	Gen 0	Gen 1	Allocated
Old	Jint	20	1.646 s	60000.0000	5000.0000	249.81 MB
New			813.2 ms (-51%)	60000.0000 (0%)	12437.5000 (+149%)	264.85 MB (+6%)

Jint.Benchmark.UncacheableExpressionsBenchmark

Diff	Method	N	Mean	Gen 0	Gen 1	Allocated
Old	Benchmark	500	461.2 ms	52812.5000	62.5000	211.35 MB
New			428.8 ms (-7%)	54812.5000 (+4%)	5875.0000 (+9300%)	220.71 MB (+4%)