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-parser/src/parse_keywords.rs

@@ -1179,10 +1179,7 @@ pub fn parse_export_in_module(
         error = error.or_else(|| {
             Some(ParseError::MissingPositional(
                 "def, def-env, alias, or env keyword".into(), // TODO: keep filling more keywords as they come
-                Span {
-                    start: export_span.end,
-                    end: export_span.end,
-                },
+                Span::new(export_span.end, export_span.end),
                 "'def', `def-env`, `alias`, or 'env' keyword.".to_string(),
             ))
         });
@@ -1472,11 +1469,10 @@ pub fn parse_module(
         if block_bytes.ends_with(b"}") {
             end -= 1;
         } else {
-            error =
-                error.or_else(|| Some(ParseError::Unclosed("}".into(), Span { start: end, end })));
+            error = error.or_else(|| Some(ParseError::Unclosed("}".into(), Span::new(end, end))));
         }
 
-        let block_span = Span { start, end };
+        let block_span = Span::new(start, end);
 
         let (block, module, err) =
             parse_module_block(working_set, block_span, expand_aliases_denylist);