improve GetRegistrationResult

[bollhals]

PR 7 extracted out of #40

Focused on the GetRegistrationResult method

Main change:

• Simplified GetRegistrationResult NamedInstance handling
• Move Implicit registration into GetRegistrationResult to avoid returning nullable KeyValuePair.

Performance measurements:

ResolveFromType

Method	Mean	Error	StdDev	Min	Max	Median	Rank	Gen 0	Gen 1	Gen 2	Allocated
Current	225.6 ns	0.35 ns	0.33 ns	225.0 ns	226.1 ns	225.5 ns	1	0.0730	-	-	344 B
Master	230.4 ns	0.90 ns	0.80 ns	229.2 ns	231.9 ns	230.3 ns	1	0.0730	-	-	344 B

It's not simply possible to write a performance test for the NamedInstance / Implicit Regisration case, as it will changes the state of the container, thus only go through that case once.