Review volatile memory about stuffer_shack-rs

[marc-casperlabs]

After some advice from the Rust Discord, the code still has issues:

stuffer_shack-rs/src/unchecked_cast.rs

Lines 51 to 56 in a6410cd

	let slice = &self[offset..(offset + mem::size_of::<T>())];
	let item_ptr = slice.as_ptr() as *const T;
	let item = unsafe { &*item_ptr };
	item

• Unaligned access must be done through ptr::read_unaligned & similar. It is probably necessary to implement #14 and in the constructor ensure that the chosen alignment is divisible by mem::align_of::<RecordHeader> when offset by the length
• There must not exist any (even temporary) aliased references to mutable memory at the same time, like there is with slice and item in the mutable version of the function. Better to do pointer math instead.
• A check for the enforced alignment should also be done, as the page boundary needs to be divisible by it - as that's the alignment that will be returned by mmap usually.

Volatile itself does not seem to be an issues, as it is mainly concerned about ensuring that multiple repeated writes to a location are not reordered or omitted, as would be the case for an IO device.