Leverage `core::marker::Freeze`

[jswrenn]

With rust-lang/rust#121501, Rust may expose and subsequently stabilize the Freeze trait, which is automatically implemented for all types free of UnsafeCell (excluding by indirection such as &UnsafeCell). This is analogous to our NoCell trait, but our trait must be explicitly derived.

Once Freeze is stabilized, we should first emit a

where
     Self: Freeze

bound on the definition of NoCell when zerocopy is compiled with sufficiently-recent toolchains.

Then, we should consider whether to eliminate our NoCell trait altogether in favor of re-exporting Freeze as NoCell.

[joshlf]

We've decided that our Immutable will need to have slightly different semantics than stdlib Freeze - namely, it bans interior mutability recursively (ie, even via reference/pointer indirection), while Freeze does not. See #1155.