refactor: simplify iterator.Try and remove unnecessary backtracking (#47)

## Description

`iterator.Try` previously copied the entire iterator and synced it back on success, causing an unnecessary heap allocation on every call.
This PR simplifies `Try` to save and restore the index directly, and removes the now-unused `Copy` and `Sync` methods.

- Rewrite `ScanRune` and `ParseRawToken` as peek-then-advance, eliminating the need for `Try` at leaf level.
- Remove redundant `Try` wrappers from `parseExpression`, `parseAbstraction`, `parseApplication`, `parseLet`, and `parseToken`, which are already disambiguated by their callers.
- Keep `Try` only where true backtracking is needed: `parseStatement`, which must choose between `parseLet` and `parseDeclare`.
- Fix pre-existing panic in saccharine `parseExpression` when the iterator is exhausted (added `Done()` guard).

### Decisions

- `Try` now operates on the original iterator instead of a copy, removing the confusing pattern where the callback's `i` was a different object than the caller's `i`.
- Removed `parseSoftBreak` and `parseHardBreak` helper functions since `ParseRawToken` no longer needs `Try` wrapping.

## Benefits

- Eliminates a heap allocation per `Try` call.
- Reduces nesting and indirection in all parse functions.
- Makes the code easier to follow by removing the shadow-`i` pattern.
- `Try` is now only used at genuine choice points in the grammar.

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`). Always use underscores.
- [x] Tests pass (if applicable).
- [x] Documentation updated (if applicable).

Reviewed-on: #47
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

pkg/token/scan.go

@@ -17,15 +17,15 @@ func IsVariable(r rune) bool {
 // predicate.
 // Returns an error if the iterator is exhausted or the rune does not match.
 func ScanRune(i *iterator.Iterator[rune], expected func(rune) bool) (rune, error) {
-	return iterator.Try(i, func(i *iterator.Iterator[rune]) (rune, error) {
-		if r, err := i.Next(); err != nil {
-			return r, err
-		} else if !expected(r) {
-			return r, fmt.Errorf("got unexpected rune %v'", r)
-		} else {
-			return r, nil
-		}
-	})
+	r, err := i.Get()
+	if err != nil {
+		return r, err
+	}
+	if !expected(r) {
+		return r, fmt.Errorf("got unexpected rune %v'", r)
+	}
+	i.Forward()
+	return r, nil
 }
 
 // ScanCharacter consumes the next rune from the iterator if it matches the