dillon1000/react

Merge consecutive reactive scopes that invalidate together

This is something we've wanted to do for a while, and which @sophiebits also 
brought up. The idea is to merge consecutive reactive scopes that will always 
invalidate together. There are two cases of this: 

* Both scopes have the exact same inputs 

* Or the inputs of the second scope are the outputs of the first 

In both these cases it's pure overhead to keep the scopes separate. In the first 
case where both scopes have the same inputs, merging allows us to check the 
inputs once instead of 2+ times. In the second case, we know that the second 
scope will invalidate when the first does so it's wasteful to recheck the 
outputs of the first scope for changes. 

This is already cutting down on memoization quite a bit in the fixtures. Note 
that there's an additional optimization we can do after merging the second 
category, which is to remove the first scope's declarations if they were only 
referenced by the second scope. I'll add that in a follow-up.