Refactor rac_sequence tests to use rac_signal instead
SHANE LONG
Merge branch 'deprecate-sequences' into subscriber-disposal
Add RACLiveSubscriber, reduce subscriber allocations

Since RACLiveSubscriber fulfills the use cases of _both_ RACSubscriber
and RACPassthroughSubscriber, we can avoid creating _two_ subscribers
for every block-based subscription, and just reuse the same object
instead.