Optimize FixedArray8<T> indexer

[sharwell]

• Use Unsafe.Add for direct access
• Apply MethodImplOptions.AggressiveInlining
• Use throw helper

Benchmark using AddRange

0. Baseline
1. Switch to Unsafe.Add in indexer
2. Add AggressiveInlining and throw helper

Method	Count	Mean	Error	StdDev	Ratio	RatioSD	Gen 0/1k Op	Gen 1/1k Op	Gen 2/1k Op	Allocated Memory/Op
ImmutableList	10	734.9 ns	393.880 ns	21.5899 ns	1.00	0.00	1.0605	-	-	834 B
ImmutableArray	10	308.4 ns	220.845 ns	12.1052 ns	0.42	0.02	0.3772	-	-	297 B
ImmutableTreeList 0	10	429.7 ns	148.300 ns	8.1290 ns	0.60	0.02	0.5097	-	-	401 B
ImmutableTreeList 1	10	288.7 ns	4.789 ns	0.2625 ns	0.39	0.01	0.5097	-	-	401 B
ImmutableTreeList 2	10	264.2 ns	14.590 ns	0.7998 ns	0.37	0.01	0.5097	-	-	401 B
ImmutableList	1000	32,815.0 ns	39,520.030 ns	2,166.2262 ns	1.00	0.00	52.0630	8.6670	-	60777 B
ImmutableArray	1000	6,758.1 ns	309.823 ns	16.9824 ns	0.21	0.01	15.8691	-	-	12508 B
ImmutableTreeList 0	1000	58,769.1 ns	11,984.860 ns	656.9300 ns	1.83	0.10	14.8926	-	-	11724 B
ImmutableTreeList 1	1000	38,737.7 ns	11,470.392 ns	628.7309 ns	1.18	0.06	14.8926	-	-	11724 B
ImmutableTreeList 2	1000	33,213.6 ns	8,506.380 ns	466.2632 ns	1.02	0.07	14.8926	-	-	11724 B
ImmutableList	100000	8,297,334.1 ns	1,368,147.755 ns	74,992.7951 ns	1.00	0.00	1000.0000	609.3750	390.6250	6265724 B
ImmutableArray	100000	718,363.3 ns	533,829.695 ns	29,261.0069 ns	0.09	0.00	399.4141	399.4141	399.4141	1452144 B
ImmutableTreeList 0	100000	10,210,487.3 ns	2,561,610.010 ns	140,410.4880 ns	1.21	0.08	187.5000	93.7500	-	1123347 B
ImmutableTreeList 1	100000	6,470,338.1 ns	1,207,622.535 ns	66,193.8661 ns	0.78	0.01	179.6875	85.9375	-	1123357 B
ImmutableTreeList 2	100000	5,601,348.3 ns	591,887.890 ns	32,443.3726 ns	0.63	0.06	179.6875	85.9375	-	1123357 B
ImmutableList	10000000	1,420,494,400.0 ns	188,008,648.079 ns	10,305,388.4080 ns	1.00	0.00	78000.0000	28000.0000	1000.0000	655634848 B
ImmutableArray	10000000	108,571,620.0 ns	81,204,808.000 ns	4,451,109.5398 ns	0.08	0.00	1400.0000	1400.0000	1400.0000	174226376 B
ImmutableTreeList 0	10000000	1,545,463,533.3 ns	160,400,634.270 ns	8,792,100.0120 ns	1.09	0.01	18000.0000	6000.0000	-	112176752 B
ImmutableTreeList 1	10000000	986,178,366.7 ns	336,117,471.221 ns	18,423,732.7752 ns	0.69	0.01	18000.0000	6000.0000	-	112176752 B
ImmutableTreeList 2	10000000	867,280,133.3 ns	155,179,711.470 ns	8,505,923.6161 ns	0.61	0.01	18000.0000	6000.0000	-	112176752 B

[codecov]

Codecov Report

Merging #102 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #102   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          169       170    +1     
  Lines        20526     20506   -20     
  Branches      1614      1614           
=========================================
- Hits         20526     20506   -20