Implement `IntoBytes` for `Cell<T: IntoBytes>`

[jswrenn]

I believe we can implement IntoBytes for Cell<T: IntoBytes>. Per Cell's documentation:

Memory layout

Cell<T> has the same memory layout and caveats as UnsafeCell. In particular, this means that Cell has the same in-memory representation as its inner type T.

— from https://doc.rust-lang.org/std/cell/struct.Cell.html#memory-layout

[jswrenn]

It looks like we can essentially copy the implementations of TransparentWrapper, TryFromBytes and other traits for UnsafeCell, but I can't quite make our TryFromBytes testing infra happy.