IsSingletonActor returns true if the actor is a singleton actor (i.e., cannot be constructed).

DebugLevel logs are typically voluminous, and are usually disabled in production.

ErrorLevel logs are high-priority.

InfoLevel is the default logging priority.

WarnLevel logs are more important than Info, but don't need individual human review.

VMActor is a concrete implementation of an actor, to be used by a Filecoin VM.

Specifies importance of message, LogLevel numbering is consistent with the uber-go/zap package.