Fix #838

- rewrite the unsafe block in `expand_process` at
  `src/lib/parser/shell_expand/mod.rs` so that strings start with whitespace
  will not cause panic. Also the definition of whitespace now includes unicode
  whitespaces (so the behavior matches `split_whitespace` which is used
  in other places), and the time complexity is reduced from O(n^2) to O(n).
- add a test in example/issues.ion for this issue