Allow creation of Spock mocks in traits

I tried to refactor some common setup code into a trait, but it appears that (as of 1.0/2.4) calling Mock() inside a trait is not supported:

Caused by: org.spockframework.runtime.InvalidSpecException: Mock objects can only be created inside a spec, and only during the lifetime of a feature (iteration)
    at spock.lang.MockingApi.invalidMockCreation(MockingApi.java:1233)
    at spock.lang.MockingApi.Mock(MockingApi.java:142)
    at spock.lang.MockingApi$Mock.call(Unknown Source)
    at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:117)
    at my.trait.MyTrait$Trait$Helper.$init$(MocksStripe.groovy:22)

I get this error regardless of whether the Mock() call is in a field initializer or in the setup() method, though it's thrown during spec initialization in the former case and on execution in the latter.

Start at spock.lang.MockingApi.invalidMockCreation and the Mock entry point cited in MockingApi.java, then reproduce the report using a trait field initializer and setup() call like MocksStripe.groovy:22. Trace how trait initialization and feature lifetimes are detected; done means valid Mock() calls in both trait contexts work without breaking the existing spec-only lifecycle rules.