This commit adjusts how exprs and for loops are handled within
rsx. This is a breaking change in terms of codegen, but has
slight semantic changes as well.
Now, when exprs/for loops are allocated, they are given a temporary.
The temporary is elided to the <'a> lifetime of the bump, to satisfy
the borrow checker. This fixes issues with signals where exprs/for
loops mapping vecs out of RefCells would be caught up without a
temporary lifetime.
Jonathan Kelley