unify read_to_end and io::copy impls for reading into a Vec This ports over the initial probe (to avoid allocation) and the dynamic read sizing from the io::copy specialization to the `default_read_to_end` implementation which already had its own optimizations for different cases. I think it should be a best-of-both now. suggested by `@a1phyr` in https://github.com/rust-lang/rust/pull/117576#issuecomment-1803408492