Add ByteVector::toUInt24() function

This issue intends to hear some second opinion.

I made the patch #154 to get rid of small-sized ```ByteVector::mid()``` opeartions. I believe that the patch itself is useful, but somehow rough. 
In the patch, ```ByteVector::toUInt()``` has an overload takes ```length``` specifies the data length that will be converted. It sounds meaningless, but I made it because some formats require to handle 24-bit integers. 
However, I now think that it is more straightforward to remove the overload and create special overloads for 24-bit integers like this patch.

Which way is better?