perf: enable clippy::large_futures lint

Large size of Mimefactor.render() futures
increases the size of all callers
down to set_config() and background_fetch().
I also had to Box::pin one call to fetch_new_msg_batch
and one call to fetch_single_msg.

src/lib.rs

@@ -17,6 +17,7 @@
     clippy::cloned_instead_of_copied,
     clippy::manual_is_variant_and
 )]
+#![cfg_attr(not(test), warn(clippy::large_futures))]
 #![cfg_attr(not(test), warn(clippy::arithmetic_side_effects))]
 #![cfg_attr(not(test), forbid(clippy::indexing_slicing))]
 #![cfg_attr(not(test), forbid(clippy::string_slice))]