`Rc` and `RefCell` both incur runtime space costs. In addition, `RefCell` has given us some headaches with the non obvious borrow panics it likes to throw out. The latest one started with 7fd53eeb46db261bbc20025cdab70096245a5cbe and is yet to be resolved. Since we already rely on the GC to properly reclaim memory for `Block` and `Branch`, we might as well stop paying the overhead of `Rc` and `RefCell`. The `RefCell` panics go away with this change, too. On 25 iterations of `railsbench` with a stats build I got `yjit_alloc_size: 8,386,129 => 7,348,637`, with the new memory size 87.6% of the status quo. This makes the metadata and machine code size roughly line up one-to-one. The general idea here is to use `&` shared references with [interior mutability][1] with `Cell`, which doesn't take any extra space. The `noalias` requirement that `&mut` imposes is way too hard to meet and verify. Imagine replacing places where we would've gotten `BorrowError` from `RefCell` with Rust/LLVM miscompiling us due to aliasing violations. With shared references, we don't have to think about subtle cases like the GC _sometimes_ calling the mark callback while codegen has an aliasing reference in a stack frame below. We mostly only need to worry about liveness, with which the GC already helps. There is now a clean split between blocks and branches that are not yet fully constructed and ones that are "in-service", so to speak. Working with `PendingBranch` and `JITState` don't really involve `unsafe` stuff. This change allows `Branch` and `Block` to not have as many optional fields as many of them are only optional during compilation. Fields that change post-compilation are wrapped in `Cell` to facilitate mutation through shared references. I do some `unsafe` dances here. I've included just a couple tests to run with Miri (`cargo +nightly miri test miri`). We can add more Miri tests if desired. [1]: https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html