Improve performance / allocations of RegisterAs* methods

[bollhals]

PR 5 extracted out of #40

Focused on the RegisterAs methods

Main change:

• NamedInstanceDictionaryRegistration changed to a singleton as it contains no state
• Avoid calling TryRemove when we're anyway just set it to a new value anyway
• A bit of cleanup and reordering to simplify what the difference between the different RegisterAs methods are

Performance measurements:

RegisterTypeAsName

Method	Mean	Error	StdDev	Min	Max	Median	Ratio	Rank	Gen 0	Gen 1	Gen 2	Allocated
Current	627.5 ns	1.79 ns	1.58 ns	625.4 ns	630.0 ns	627.6 ns	1.05	2	0.0696	-	-	328 B
Master	711.1 ns	2.84 ns	2.52 ns	707.7 ns	715.9 ns	711.1 ns	1.23	2	0.0858	-	-	408 B

RegisterTypeAs

Method	Mean	Error	StdDev	Min	Max	Median	Ratio	RatioSD	Rank	Gen 0	Gen 1	Gen 2	Allocated
Current	88.83 ns	1.241 ns	1.160 ns	87.36 ns	90.64 ns	88.38 ns	1.23	0.03	2	0.0204	-	-	96 B
Master	146.97 ns	1.455 ns	1.361 ns	144.89 ns	148.69 ns	147.37 ns	2.06	0.02	2	0.0322	-	-	152 B

RegisterInstance

Method	Mean	Error	StdDev	Min	Max	Median	Ratio	RatioSD	Rank	Gen 0	Gen 1	Gen 2	Allocated
Current	126.6 ns	0.88 ns	0.82 ns	125.5 ns	127.9 ns	126.4 ns	1.14	0.01	2	0.0186	-	-	88 B
Master	184.4 ns	1.26 ns	1.18 ns	182.8 ns	186.6 ns	184.4 ns	1.66	0.02	2	0.0305	-	-	144 B