NLS is Non-lexical Scopes for Go

Go has defer which gives something approaching strict lexical lifetimes for resource management but there is no direct support for object or resource lifetimes that may outlive the function in which they are created. This small library allows the user to create a tree of scope instances, each of which owns the lifetime of zero or more resource-consuming objects (e.g. goroutines, sockets, gRPC clients, etc) and which can be used to cleanly release said resources.