Drop DecimalArithmetic

[michalmuskala]

It wasn't actually used anywhere.

Also fix requirement in decimal - Decimal.cmp was introduced in 1.2

[coveralls]

Pull Request Test Coverage Report for Build 174

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 173:	0.0%
Covered Lines:	111
Relevant Lines:	111

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💛 - Coveralls

[danielberkompas]

Thank you! @michalmuskala