Because many Reader/Writer implementations (e.g. bufio) assume their
able to use subarray() with byte indexes and often ask for byte values,
it makes sense to simply restrict all implementations to Uint8Array.
This will improve the threshold benchmark. Using 32k because that's what
Go uses, but we should explore the value in the future.
a0d6420d8b/src/io/io.go (L391)