only assert in debug builds

these are primarily used to detect bugs during e.g. fuzzing.

most of these asserts have negligible impact on performance, but if they
are not debug asserts it is not obvious that they dont affect
performance of release builds

src/lib.rs

@@ -993,7 +993,7 @@ impl<'s> Parser<'s> {
                                 class: (!ev.span.is_empty()).then(|| content),
                             },
                             block::Container::Footnote => {
-                                assert!(enter);
+                                debug_assert!(enter);
                                 self.footnotes.insert(content, self.tree.take_branch());
                                 self.block_attributes = Attributes::new();
                                 continue;