Audit uses of `addr_of!` for read-only-ness

[joshlf]

Confirm that our uses of addr_of! are sound in light of rust-lang/rust#129653. As of this writing, it appears that there are two uses:

One is Unalign::get_ptr:

zerocopy/src/wrappers.rs

Lines 233 to 235 in b8c3178

	pub const fn get_ptr(&self) -> *const T {
	ptr::addr_of!(self.0)
	}

One is in trailing_field_offset!:

zerocopy/src/macro_util.rs

Lines 187 to 189 in b8c3178

	let field = unsafe {
	$crate::macro_util::core_reexport::ptr::addr_of!((*ptr).$trailing_field_name)
	};

The Unalign::get_ptr one may be problematic if the user uses the returned *const T to perform mutation. We may need to clarify in the safety docs on that method that the returned pointer is read-only.