Fix footnote definition performance.

We were re-parsing each footnote definition in the footnote definition exit matcher which causes their contents to get re-parsed. This compounds with long lists of footnote definitions.

src/parser/regular_link.rs

@@ -3,7 +3,6 @@ use nom::bytes::complete::escaped;
 use nom::bytes::complete::tag;
 use nom::bytes::complete::take_till1;
 use nom::character::complete::one_of;
-
 use nom::combinator::verify;
 use nom::multi::many_till;