O(n^2) for parsing <><><><><>...

[mity]

$ time python -c 'print("<>" * 10000)' | ./target/release/comrak >/dev/null
real    0m0.394s
user    0m0.392s
sys     0m0.002s

$ time python -c 'print("<>" * 20000)' | ./target/release/comrak >/dev/null
real    0m1.549s
user    0m1.547s
sys     0m0.003s

$ time python -c 'print("<>" * 40000)' | ./target/release/comrak >/dev/null
real    0m6.184s
user    0m6.180s
sys     0m0.004s