Replace DebugLog with Microsoft.Extensions.Logging

[yaakov-h]

Since there's now a fairly standard .NET method of logging - or at least a standard set of interfaces for multiple backends, we don't really need our own.

If we can, only take a dependency on Microsoft.Extensions.Logging.Abstractions and let the consumer deal with setup and implementation, if they even care to enable logging.

Our samples (3.DebugLog) should provide a reasonable default, probably AddConsole().

Try not to take a dependency on MS DI if we don't need to.

See also: dotnet/extensions#615 (comment)