Different singleton registrations resolve to same instance

[yufeih]

• Singleton uses same instance when registered with different service types
• Scoped instances are same within a scope but are different across scopes
• Singleton instances are disposed exactly once

#360

[dnfclas]

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[davidfowl]

This is a breaking change. If we decide to support this, I'm not sure we can just break the existing behavior. It may need to be a different registration syntax

[yufeih]

I see, like services.AddSingleton<IBar, IBaz, Bar>();

[Eilon]

Closing per #360 (comment)