dillon1000/react

[be] Limit scope of temporaries in codegen

This morning @mofeiZ reminded me that our codegen doesn't really have any guards 
against reordering, since temporaries are lazily emitted. We're relying on the 
fact that our lowering and memoization carefully preserves order of evaluation, 
such that delaying the instructions in codegen doesn't change semantics. 

To help catch any mistakes with this, I had previously added code that reset the 
codegen context's temporaries before/after exiting a reactive scope. That 
ensured that temporaries from within the scope weren't accessible outside it. 
This PR extends that approach to _all_ blocks, so that temporaries created 
within a block aren't accessible outside it. 

I'm also going to explore more actively resetting temporaries after they 
"should" be used. There are a couple cases where temporaries are reused, though, 
which we have to change first.