Add StringSegment benchmarks

[gfoidl]

BenchmarkDotNet=v0.10.11, OS=ubuntu 17.10
Processor=Intel Xeon CPU 2.60GHz, ProcessorCount=2
.NET Core SDK=2.1.4
  [Host]     : .NET Core 2.0.5 (Framework 4.6.0.0), 64bit RyuJIT
  Job-NYQHGM : .NET Core 2.0.5 (Framework 4.6.0.0), 64bit RyuJIT

RemoveOutliers=False  Runtime=Core  Server=True  
Toolchain=.NET Core 2.0  LaunchCount=3  RunStrategy=Throughput  
TargetCount=10  WarmupCount=5  

Method	Mean	Error	StdDev	Op/s	Gen 0	Allocated
Ctor_String	6.1938 ns	0.0558 ns	0.0836 ns	161,450,947.8	-	0 B
GetValue	3.2986 ns	0.1685 ns	0.2522 ns	303,154,762.2	-	0 B
Indexer	0.2526 ns	0.0091 ns	0.0136 ns	0.2496 ns	3,958,731,026.6	-
Equals_Object_Invalid	3.4613 ns	0.0601 ns	0.0900 ns	288,912,359.3	-	0 B
Equals_Object_Valid	11.8679 ns	0.1978 ns	0.2960 ns	84,260,973.3	-	0 B
Equals_Valid	6.0853 ns	0.0787 ns	0.1178 ns	164,331,267.7	-	0 B
Equals_String	8.1777 ns	0.0236 ns	0.0353 ns	122,284,384.5	-	0 B
GetHashCode	21.9225 ns	0.0661 ns	0.0989 ns	45,615,211.1	-	0 B
StartsWith	15.9217 ns	0.0959 ns	0.1436 ns	62,807,179.4	-	0 B
EndsWith	15.7556 ns	0.0701 ns	0.1049 ns	63,469,480.2	-	0 B
SubString	21.7661 ns	0.1461 ns	0.2186 ns	45,943,104.2	0.0005	32 B
SubSegment	26.0086 ns	0.5708 ns	0.8544 ns	38,448,752.4	-	0 B
IndexOf	10.7048 ns	0.2679 ns	0.4010 ns	93,416,223.0	-	0 B
IndexOfAny	21.2379 ns	0.5609 ns	0.8395 ns	47,085,709.4	-	0 B
LastIndexOf	10.3372 ns	0.3383 ns	0.5064 ns	96,737,556.7	-	0 B
Trim	79.6639 ns	1.2935 ns	1.9361 ns	12,552,736.8	-	0 B
TrimStart	42.6913 ns	1.0775 ns	1.6128 ns	23,423,963.4	-	0 B
TrimEnd	44.2018 ns	0.7579 ns	1.1344 ns	22,623,514.0	-	0 B

[gfoidl]

/cc @rynowak

[davidfowl]

Can we add a bigger string in one of them

[gfoidl]

Added a larger one -> 6dfaf3f

Updated the numbers in the description based on the current code for the benchmarks.