perf: remove .collect() into Vec in respan_token_stream

[dishmaker]

427 less llvm lines

Before:

syn$ cargo llvm-lines --all-features --release | head -n 3
  Lines                 Copies              Function name
  -----                 ------              -------------
  221565                4748                (TOTAL)

After:

syn$ cargo llvm-lines --all-features --release | head -n 3
  Lines                 Copies              Function name
  -----                 ------              -------------
  221138                4723                (TOTAL)