perf: remove .collect() to Vec in TokenStreamHelper

[dishmaker]

608 less llvm lines.

Before:

syn$ cargo llvm-lines --all-features --release | head -n 3
  Lines                 Copies              Function name
  -----                 ------              -------------
  225535                4804                (TOTAL)

After:

syn$ cargo llvm-lines --all-features --release | head -n 3
  Lines                 Copies              Function name
  -----                 ------              -------------
  224927                4790                (TOTAL)

[dishmaker]

Current master:

syn$ cargo llvm-lines --release --all-features | head -n 3
   Compiling quote v1.0.41 (../quote)
   Compiling syn v2.0.108
    Finished `release` profile [optimized] target(s) in 11.43s
  Lines                 Copies              Function name
  -----                 ------              -------------
  219848                4642                (TOTAL)

syn$ cargo llvm-lines --release | head -n 3
   Compiling syn v2.0.108
    Finished `release` profile [optimized] target(s) in 4.52s
  Lines                 Copies              Function name
  -----                 ------              -------------
  108228                2609                (TOTAL)

syn$ cargo -V
cargo 1.93.0-nightly (636800288 2025-10-31)

syn$ git log -1
commit 1893ecddd87bf789c41e6feef4d607f0f4064eb6 (HEAD -> master, origin/master, origin/HEAD)
Author: David Tolnay <[email protected]>
Date:   Mon Nov 3 13:25:45 2025 -0800

    Touch up PR 1933