dillon1000/react

[rust] Remove arena allocator

Using an arena allocator can be faster, but it comes with several challenges: 

* It requires tediously tagging nearly every value with a lifetime. Any function 
that has to deal with arena-allocated data (which is every meaningful function) 
ends up with a lifetime parameter. Bleh. We also have to thread the allocator 
itself wherever we need to allocate, though this is less of a problem since 
_most_ functions already need the Environment and we can store the allocator 
there. 

* There is not yet widespread support in the Rust ecosystem for using custom 
allocators with custom data types. This means that things like HashMap/Set and 
IndexMap/Set can't be arena allocated. This means that either we have to add 
support to these data types (by upstreaming or forking) or just accept that 
we're only partially using the arena allocator. 

* Finally, `bumpalo`'s `Box` cannot be moved out of, which turns out to be an 
annoying limitation that i've already had to work around several times. 

In the end i'm not sure arena allocators are worth it at this stage of the 
project. Relay Compiler has been successful without one, and other Rust-based 
internal compilers get by without them too.