[lib] reorder_line(): Expand LTR-only shortcut condition

* There's almost no extra cost in expanding the shortcut condition to
check all the isolating run sequences to be LTR, if there's more than
one. This can prevent a bunch of string manipulation when all isolating
run sequences are LTR.

Benchmark is unchanged:
```
Running target/release/deps/basic-e727a8b30b644dba

running 4 tests
test bench_1_bidi_info_new_for_ltr_texts  ... bench:       1,766 ns/iter (+/- 291)
test bench_2_bidi_info_new_for_bidi_texts ... bench:       2,158 ns/iter (+/- 238)
test bench_3_reorder_line_for_ltr_texts   ... bench:         250 ns/iter (+/- 63)
test bench_4_reorder_line_for_bidi_texts  ... bench:         594 ns/iter (+/- 72)

Running target/release/deps/udhr-c7491ea72b980ef8

running 4 tests
test bench_1_bidi_info_new_for_ltr_texts  ... bench:     667,871 ns/iter (+/- 114,556)
test bench_2_bidi_info_new_for_bidi_texts ... bench:     829,440 ns/iter (+/- 95,813)
test bench_3_reorder_line_for_ltr_texts   ... bench:     107,855 ns/iter (+/- 12,334)
test bench_4_reorder_line_for_bidi_texts  ... bench:     367,103 ns/iter (+/- 48,976)
```

Author: behnam

src/lib.rs

@@ -313,7 +313,9 @@ impl<'text> BidiInfo<'text> {
     /// Re-order a line based on resolved levels and return the line in display order.
     pub fn reorder_line(&self, para: &ParagraphInfo, line: Range<usize>) -> Cow<'text, str> {
         let (levels, runs) = self.visual_runs(para, line.clone());
-        if runs.len() == 1 && levels[runs[0].start].is_ltr() {
+
+        // If all isolating run sequences are LTR, no reordering is needed
+        if runs.iter().all(|run| levels[run.start].is_ltr()) {
             return self.text[line.clone()].into();
         }