Better IO buffer when validating dist hashes
Better IO buffer when validating dist hashes

Also for a bonus, show a progress bar at the same time.

rust-lang/rustup,nodakai/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,nodakai/rustup.rs,inejge/rustup.rs,inejge/rustup.rs,rust-lang/rustup,inejge/rustup.rs,rust-lang/rustup,inejge/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,inejge/rustup.rs