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/subscript_and_superscript.rs

@@ -2,7 +2,6 @@ use nom::branch::alt;
 use nom::bytes::complete::tag;
 use nom::character::complete::anychar;
 use nom::character::complete::one_of;
-
 use nom::combinator::map;
 use nom::combinator::not;
 use nom::combinator::opt;