Rollup merge of rust-lang#70910 - rakshith-ravi:master, r=cuviper

Hides default fns inside Fuse impl to avoid exposing it to any crate

Fixes rust-lang#70796

@cuviper I've added some default, private traits to do the job for us. If required, I can expose them to a specific visibility if you want to call these functions for rust-lang#70332

r? @cuviper