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/tests/test_lex.rs

@@ -18,7 +18,7 @@ fn lex_newline() {
 
     assert!(output.0.contains(&Token {
         contents: TokenContents::Eol,
-        span: Span { start: 11, end: 12 }
+        span: Span::new(11, 12)
     }));
 }
 
@@ -43,7 +43,7 @@ fn lex_parenthesis() {
         output.0.get(3).unwrap(),
         &Token {
             contents: TokenContents::Item,
-            span: Span { start: 8, end: 27 }
+            span: Span::new(8, 27)
         }
     );
 }
@@ -58,7 +58,7 @@ fn lex_comment() {
         output.0.get(4).unwrap(),
         &Token {
             contents: TokenContents::Comment,
-            span: Span { start: 12, end: 24 }
+            span: Span::new(12, 24)
         }
     );
 }
@@ -108,14 +108,14 @@ fn lex_comments() {
         output.0.get(4).unwrap(),
         &Token {
             contents: TokenContents::Comment,
-            span: Span { start: 10, end: 19 }
+            span: Span::new(10, 19)
         }
     );
     assert_eq!(
         output.0.get(5).unwrap(),
         &Token {
             contents: TokenContents::Eol,
-            span: Span { start: 19, end: 20 }
+            span: Span::new(19, 20)
         }
     );
 
@@ -125,14 +125,14 @@ fn lex_comments() {
         output.0.get(10).unwrap(),
         &Token {
             contents: TokenContents::Comment,
-            span: Span { start: 31, end: 40 }
+            span: Span::new(31, 40)
         }
     );
     assert_eq!(
         output.0.get(11).unwrap(),
         &Token {
             contents: TokenContents::Eol,
-            span: Span { start: 40, end: 41 }
+            span: Span::new(40, 41)
         }
     );
 }