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/inline_source_block.rs

@@ -4,7 +4,6 @@ use nom::bytes::complete::tag_no_case;
 use nom::character::complete::anychar;
 use nom::character::complete::line_ending;
 use nom::character::complete::one_of;
-
 use nom::combinator::opt;
 use nom::combinator::recognize;
 use nom::combinator::verify;