Cache low-numbered numeric event ids to reduce allocations

[nblumhardt]

This is a quick improvement for #118. Needs a benchmark to measure allocs.

[nblumhardt]

Just a note, most of the framework projects now user, or will use, event ids: dotnet/aspnetcore#6381

[nblumhardt]

Added some very basic, broad benchmarks. Shows (I think) that it's worth making this change, but also that there's a lot of opportunity still there in the allocations department.

BenchmarkDotNet=v0.11.5, OS=Windows 10.0.18362
Intel Core i7-7700HQ CPU 2.80GHz (Kaby Lake), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=2.2.105
  [Host]     : .NET Core 2.2.6 (CoreCLR 4.6.27817.03, CoreFX 4.6.27818.02), 64bit RyuJIT
  DefaultJob : .NET Core 2.2.6 (CoreCLR 4.6.27817.03, CoreFX 4.6.27818.02), 64bit RyuJIT

Method	Mean	Error	StdDev	Ratio	RatioSD	Gen 0	Gen 1	Gen 2	Allocated
Native	381.0 ns	41.01 ns	48.82 ns	1.00	0.00	0.0887	-	-	280 B
NoId	1,004.2 ns	10.84 ns	10.14 ns	2.63	0.34	0.2785	-	-	880 B
LowNumbered	1,367.0 ns	37.37 ns	104.80 ns	3.49	0.48	0.3529	-	-	1112 B
HighNumbered	1,378.5 ns	27.65 ns	77.08 ns	3.67	0.43	0.3777	-	-	1192 B