Stop making subprojects mutate instances

[vreuter]

#309 and many others. The mutability when working with subprojects is poor design. activate/deactivate calls should make new instance.

[vreuter]

This probably shouldn't be attempted for 0.22

[vreuter]

This was mainly motivated mainly by issues that have since been solved and/or turned out to not directly relate to subproject activation/deactivation implementation. I still don't love the way those actions are implemented (affecting the instance on which they're called and allowing an end user or client code to use the same reference), but the implied proposal would be a big API change. With no more instance mutation, callers would need to assign the result to a variable for the method call to have any meaning where it's made. @nsheff should this large significant a change wait for a 1.x release?

[nsheff]

@vreuter, I wanted to ping on this. @stolarczyk with the new version, how did you handle the amendments?