Make the tracing macros optional.

src/parser/diary_sexp.rs

@@ -13,7 +13,7 @@ use crate::parser::util::get_consumed;
 use crate::parser::util::start_of_line;
 use crate::parser::DiarySexp;
 
-#[tracing::instrument(ret, level = "debug")]
+#[cfg_attr(feature = "tracing", tracing::instrument(ret, level = "debug"))]
 pub fn diary_sexp<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s str, DiarySexp<'s>> {
     start_of_line(context, input)?;
     let (remaining, _leading_whitespace) = space0(input)?;