Use SmallVec for local stack in isolating_run_sequences().

src/prepare.rs

@@ -14,6 +14,8 @@
 use alloc::vec::Vec;
 use core::cmp::max;
 use core::ops::Range;
+#[cfg(feature = "smallvec")]
+use smallvec::{SmallVec, smallvec};
 
 use super::level::Level;
 use super::BidiClass::{self, *};
@@ -52,6 +54,9 @@ pub fn isolating_run_sequences(
 
     // When we encounter an isolate initiator, we push the current sequence onto the
     // stack so we can resume it after the matching PDI.
+    #[cfg(feature = "smallvec")]
+    let mut stack: SmallVec::<[Vec<Range<usize>>; 8]> = smallvec![vec![]];
+    #[cfg(not(feature = "smallvec"))]
     let mut stack = vec![vec![]];
 
     for run in runs {