xous has unsound usages of `core::slice::from_raw_parts`

We consider as_slice and as_slice_mut unsound because: the pointer with any bit patterns could be cast to the slice of arbitrary types. The pointer could be created by unsafe new and deprecated from_parts. We consider that from_parts should be removed in latest version because it will help trigger unsoundness in as_slice. With new declared as unsafe, as_slice should also declared as unsafe.

This was patched in by marking two functions as unsafe.