hashFile

fn hashFile(io: Io, file: Io.File, bin_digest: *[Hasher.mac_length]u8) Io.File.ReadPositionalError!void {
    var buffer: [2048]u8 = undefined;
    var hasher = hasher_init;
    var offset: u64 = 0;
    while (true) {
        const n = try file.readPositional(io, &.{&buffer}, offset);
        if (n == 0) break;
        hasher.update(buffer[0..n]);
        offset += n;
    }
    hasher.final(bin_digest);
}