O(n^2) for link reference definitions

[mity]

$ time python -c 'print("[a]: u\n" * 10000)' | ./target/release/comrak >/dev/null
real    0m0.166s
user    0m0.164s
sys     0m0.003s

$ time python -c 'print("[a]: u\n" * 20000)' | ./target/release/comrak >/dev/null
real    0m0.631s
user    0m0.631s
sys     0m0.001s

$ time python -c 'print("[a]: u\n" * 40000)' | ./target/release/comrak >/dev/null
real    0m2.482s
user    0m2.481s
sys     0m0.002s