Clean up arithmetic dimensionality

[HadrienG2]

Doing arithmetic between ByteSizes and raw integers requires a little bit of care

• Adding together ByteSizes and raw integers should be disallowed, as it encourages people to work with raw byte counts instead of explicitly stating what they mean.
• Multiplication of ByteSizes and integers should be commutative.

This was discussed in issue #2, and this MR implements the proposed set of arithmetic operators.

[hyunsik]

I'm sorry for a late response. As you suggested, I also cleaned up the arithmetic operators. You can see the improvement at https://github.com/hyunsik/bytesize/releases/tag/release-0.2.0.