Add way to set default scope

[huguesb]

If a binding exists for the Scoping class, use it whenever no
explicit scope is provided.

NB: Only instance bindings are supported at this time.

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[sameb]

Thanks for this PR, however I don't think we want to let folks change the default scope. Also, Scoping is an internal class that users should not be using.

[huguesb]

I understand the concern about the use of Scoping. Is there any chance a different patch could be accepted?

We have a sizable code base where, for various historical and technical reasons, most injected classes are singletons. Having to annotate each and every class is tedious and error-prone so we've been using a custom build of guice with this patch for more than two years and I hoped the 4.0 release window would be a good opportunity to switch back to a vanilla build.

[sameb]

Allowing folks to easily change the default scope makes using Guice unpredictable -- modules & code would be written for one paradigm & then accidentally used in another.

Keep in mind https://github.com/google/guice/wiki/Scopes#choosing-a-scope -- "If the object is stateless and inexpensive to create, scoping is unnecessary. Leave the binding unscoped and Guice will create new instances as they're required." ... It's likely that you don't really need all your classes as singletons, so consider re-evaluating that. Only stateful objects or objects that are expensive to create (or created so many times as to be a perf problem) really need to be singletons.