add node.data() and node.data_mut() shorthand.

[kivikakk]

This also makes it easy to experiment with swapping out RefCell. Hrm. Why would someone want to do that? :)

[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./bench.sh ./comrak-abfb721	108.9 ± 6.7	104.8	137.9	1.84 ± 0.12
./bench.sh ./comrak-main	107.0 ± 1.1	105.3	110.0	1.81 ± 0.03
./bench.sh ./pulldown-cmark	59.1 ± 0.8	58.0	63.0	1.00
./bench.sh ./cmark-gfm	94.6 ± 6.8	83.8	105.1	1.60 ± 0.12
./bench.sh ./markdown-it	264.3 ± 3.0	259.3	271.3	4.47 ± 0.08

Run on Tue Oct 21 10:10:43 UTC 2025