Subtract with overflow for integers

[mineichen]

There is currently no support for core::num::Wrapping(). If numbers overflow with #[cfg = debug_assertions], the program panics. It would be nice, if this library supports all of it's arithmetics for AutoSimd<[Wrapping<T>]> for all T=i8/i16/i32/i64/usize/isize/u8/u16/u32/u64 like for T itself. This seems to be the only overflow-mode in the proposal for std::simd::Simd. It would be very nice to have it in simba!

let x = AutoSimd([1, 2]);
println!("{}", x * x); // works
let x = AutoSimd([Wrapping(1), Wrapping(2)]);
println!("{}", x * x); // doesn't compile yet