Work out intended semantics of overlapping interactions about spock HOT 2 CLOSED

Finally decided to implement spreading of invocations among interactions. 
Driver was to make ordered interactions behave as expected. For example, the 
following now passes:

when:
foo.bar(1)
foo.bar(2)
foo.bar(1)

then:
1 * foo.bar(1)

then:
1 * foo.bar(2)

then:
1 * foo.bar(1)

Currently, invocations are not spread among scopes. Otherwise, code like the 
following would pass:

1 * foo.bar(1)

when:
foo.bar(1)

then:
0 * foo.bar(1)

It's safe to later change our mind on this because just like the current 
change, spreading among scopes would be a backwards compatible change.