infallible accessor methods to first and last byte

[hubcycle]

Provide the following methods for NonEmptyBz:

impl<T: AsRef<[u8]>> NonEmptyBz<T> {
	/// Returns the first byte of the contained value.
	pub fn first(&self) -> u8;

	/// Returns the last byte of the contained value.
	pub fn last(&self) -> u8;

	/// Returns the first byte and the remaining slice.
	///
	/// The remaining slice will be empty if the contained value has only one byte.
	pub fn split_first(&self) -> (u8, &[u8]);

	/// Returns the last byte and the remaining slice.
	///
	/// The remaining slice will be empty if the contained value has only one byte.
	pub fn split_last(&self) -> (u8, &[u8]);
}

unsafe code

There will be unsafe blocks to use the slice::get_unchecked method for maximum performance.

miri

Ensure all the edge cases are properly tested.