Protocol: debug_assert!() Span to reflect a valid slice (#6806)

Also enforce this by #[non_exhaustive] span such that going forward we
cannot, in debug builds (1), construct invalid spans.

The motivation for this stems from #6431 where I've seen crashes due to
invalid slice indexing.

My hope is this will mitigate such senarios

1. https://github.com/nushell/nushell/pull/6431#issuecomment-1278147241

# Description

(description of your pull request here)

# Tests

Make sure you've done the following:

- [ ] Add tests that cover your changes, either in the command examples,
the crate/tests folder, or in the /tests folder.
- [ ] Try to think about corner cases and various ways how your changes
could break. Cover them with tests.
- [ ] If adding tests is not possible, please document in the PR body a
minimal example with steps on how to reproduce so one can verify your
change works.

Make sure you've run and fixed any issues with these commands:

- [x] `cargo fmt --all -- --check` to check standard code formatting
(`cargo fmt --all` applies these changes)
- [ ] `cargo clippy --workspace --features=extra -- -D warnings -D
clippy::unwrap_used -A clippy::needless_collect` to check that you're
using the standard code style
- [ ] `cargo test --workspace --features=extra` to check that all the
tests pass

# Documentation

- [ ] If your PR touches a user-facing nushell feature then make sure
that there is an entry in the documentation
(https://github.com/nushell/nushell.github.io) for the feature, and
update it if necessary.

crates/nu-command/src/filesystem/ls.rs

@@ -518,7 +518,7 @@ pub(crate) fn dir_entry_dict(
 
         if md.is_dir() {
             if du {
-                let params = DirBuilder::new(Span { start: 0, end: 2 }, None, false, None, false);
+                let params = DirBuilder::new(Span::new(0, 2), None, false, None, false);
                 let dir_size = DirInfo::new(filename, &params, None, ctrl_c).get_size();
 
                 vals.push(Value::Filesize {

crates/nu-command/src/filesystem/rm.rs

@@ -199,18 +199,18 @@ fn rm(
         ));
     }
 
-    let targets_span = Span {
-        start: targets
+    let targets_span = Span::new(
+        targets
             .iter()
             .map(|x| x.span.start)
             .min()
             .expect("targets were empty"),
-        end: targets
+        targets
             .iter()
             .map(|x| x.span.end)
             .max()
             .expect("targets were empty"),
-    };
+    );
 
     let path = current_dir(engine_state, stack)?;