Add a test for nested Arena.alloc
Remove & -> &mut transmute from Arena
Remove & -> &mut transmute from TypedArena
auto merge of #14768 : riccieri/rust/detransmute-arena, r=alexcrichton

**Update**

I've reimplemented this using `Cell` and `RefCell`, as suggested by @alexcrichton. By taking care with the duration of the borrows, I was able to maintain the recursive allocation feature (now covered by a test) without the use of `Unsafe`, and without breaking the non-aliasing `&mut` invariant.

**Original**

Changes both `Arena` and `TypedArena` to contain an inner struct wrapped in a `Unsafe`, and change field access to go through those instead of transmuting `&self` to `&mut self`.

Part of #13933