//   - On macOS and iOS, Reader uses arc4random_buf(3).
//   - On OpenBSD and NetBSD, Reader uses getentropy(2).
//   - On other Unix-like systems, Reader reads from /dev/urandom.
//   - On Windows, Reader uses the ProcessPrng API.
//   - On js/wasm, Reader uses the Web Crypto API.
//   - On wasip1/wasm, Reader uses random_get from wasi_snapshot_preview1.
var Reader io.Reader

// Read is a helper function that calls Reader.Read using io.ReadFull.