cmark: patch allocator to accept a context object

This is necessary to be able to use arbitrary zig allocators as the
cmark allocator. I'm not sure if this patch is worth trying to
upstream.

deps/cmark/api_test/main.c

@@ -322,7 +322,7 @@ static void iterator_delete(test_batch_runner *runner) {
 
   cmark_mem *allocator = cmark_get_default_mem_allocator();
 
-  allocator->free(html);
+  allocator->free(allocator->ctx, html);
   cmark_iter_free(iter);
   cmark_node_free(doc);
 }