nodes: add From impls for AstNode.

[kivikakk]

Fixes #350 (cc @yannham).

[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./bench.sh ./comrak-4201c46	319.8 ± 2.4	317.0	324.5	2.90 ± 0.05
./bench.sh ./comrak-main	318.3 ± 1.2	316.0	321.3	2.89 ± 0.04
./bench.sh ./pulldown-cmark	110.3 ± 1.6	108.8	117.1	1.00
./bench.sh ./cmark-gfm	120.0 ± 3.7	117.4	132.3	1.09 ± 0.04
./bench.sh ./markdown-it	482.6 ± 2.5	478.6	487.7	4.38 ± 0.07

Run on Fri Jul 5 16:14:41 UTC 2024