Make the tracing macros optional.

src/parser/keyword.rs

@@ -16,7 +16,7 @@ use crate::error::Res;
 use crate::parser::util::start_of_line;
 use crate::parser::Keyword;
 
-#[tracing::instrument(ret, level = "debug")]
+#[cfg_attr(feature = "tracing", tracing::instrument(ret, level = "debug"))]
 pub fn keyword<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s str, Keyword<'s>> {
     start_of_line(context, input)?;
     // TODO: When key is a member of org-element-parsed-keywords, value can contain the standard set objects, excluding footnote references.