Traits other than NoCell permit UnsafeCells (#682)

Previously, `FromZeros`, `FromBytes`, and `AsBytes` could not be
implemented for types containing `UnsafeCell`s. This is a soundness
precondition for some types of reference transmutations (notably
transmuting either direction between `&[u8]` and `&T`). However, some of
our machinery operates only on values (e.g. `transmute!`), and that
machinery should in principle be able to support types which contain
`UnsafeCell`s.

In this commit, we remove the "no `UnsafeCell`" restriction from
`FromZeros`, `FromBytes`, and `AsBytes`. We use the recently-added
`NoCell` trait as a bound on individual functions and methods where
`UnsafeCell`s would be unsound. This permits some APIs to support
`UnsafeCell`s which could not previously support them.

Closes #251

src/byteorder.rs

@@ -35,10 +35,10 @@
 //!
 //! ```rust,edition2021
 //! # #[cfg(feature = "derive")] { // This example uses derives, and won't compile without them
-//! use zerocopy::{AsBytes, ByteSlice, FromBytes, FromZeros, Ref, Unaligned};
+//! use zerocopy::{AsBytes, ByteSlice, FromBytes, FromZeros, NoCell, Ref, Unaligned};
 //! use zerocopy::byteorder::network_endian::U16;
 //!
-//! #[derive(FromZeros, FromBytes, AsBytes, Unaligned)]
+//! #[derive(FromZeros, FromBytes, AsBytes, NoCell, Unaligned)]
 //! #[repr(C)]
 //! struct UdpHeader {
 //!     src_port: U16,
@@ -655,7 +655,7 @@ mod tests {
     use compatibility::*;
 
     // A native integer type (u16, i32, etc).
-    trait Native: Arbitrary + FromBytes + AsBytes + Copy + PartialEq + Debug {
+    trait Native: Arbitrary + FromBytes + AsBytes + NoCell + Copy + PartialEq + Debug {
         const ZERO: Self;
         const MAX_VALUE: Self;
 
@@ -692,7 +692,7 @@ mod tests {
     }
 
     trait ByteArray:
-        FromBytes + AsBytes + Copy + AsRef<[u8]> + AsMut<[u8]> + Debug + Default + Eq
+        FromBytes + AsBytes + NoCell + Copy + AsRef<[u8]> + AsMut<[u8]> + Debug + Default + Eq
     {
         /// Invert the order of the bytes in the array.
         fn invert(self) -> Self;