Fast array partition

[jackmott]

A partition function for arrays that runs about twice as fast and uses about half as much memory. This should be true for all cases, I believe.

Test results using BenchmarkDotNet, with a partition predicate of fun x -> x % 2 = 0 on random int arrays.

Host Process Environment Information:
BenchmarkDotNet=v0.9.8.0
OS=Microsoft Windows NT 6.2.9200.0
Processor=Intel(R) Core(TM) i7-4712HQ CPU 2.30GHz, ProcessorCount=8
Frequency=2240907 ticks, Resolution=446.2479 ns, Timer=TSC
CLR=MS.NET 4.0.30319.42000, Arch=64-bit RELEASE [RyuJIT]
GC=Concurrent Workstation
JitModules=clrjit-v4.6.1590.0
Type=SIMDBenchmark Mode=Throughput Platform=X64 Jit=RyuJit GarbageCollection=Concurrent Workstation 

Method	ArrayLength	Median	StdDev	Scaled	Gen 0	Gen 1	Gen 2	Bytes Allocated/Op
Partition	10	180.8758 ns	16.3650 ns	1.00	0.01	-	-	185.22
NewPartition	10	76.6145 ns	1.2114 ns	0.42	0.01	-	-	90.38
Partition	100	993.5865 ns	17.6074 ns	1.00	0.06	-	-	773.14
NewPartition	100	546.2619 ns	21.6442 ns	0.55	0.03	-	-	422.67
Partition	10000	117,268.5175 ns	1,064.2667 ns	1.00	6.40	-	-	99,742.26
NewPartition	10000	79,020.6291 ns	474.4149 ns	0.67	2.64	-	-	43,572.00
Partition	1000000	14,447,233.3078 ns	205,852.6672 ns	1.00	6.93	-	79.49	5,919,035.36
NewPartition	1000000	9,908,404.2756 ns	115,392.7494 ns	0.69	-	-	78.45	3,587,942.38
Partition	10000000	154,545,402.8213 ns	3,116,253.3692 ns	1.00	-	-	62.02	59,133,643.66
NewPartition	10000000	98,768,489.7225 ns	726,198.4079 ns	0.64	-	-	34.00	29,686,956.03

[msftclas]

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[forki]

"A partition function for arrays that runs about twice as fast and uses about half as much memory."

[jackmott]

merging latest master into this and cleaning some things up, stand by.

[jackmott]

All done here unless someone has suggestions.

[KevinRansom]

Looks great to me. I will probably pull it later this evening.

[dsyme]

Wow. Superb.