readUnitHeader

pub fn readUnitHeader(r: *Reader, endian: Endian) ScanError!UnitHeader {
    return switch (try r.takeInt(u32, endian)) {
        0...0xfffffff0 - 1 => |unit_length| .{
            .format = .@"32",
            .header_length = 4,
            .unit_length = unit_length,
        },
        0xfffffff0...0xffffffff - 1 => bad(),
        0xffffffff => .{
            .format = .@"64",
            .header_length = 12,
            .unit_length = try r.takeInt(u64, endian),
        },
    };
}