Julia: Find better names for traits TypeOrder, TypeArithmetic and TypeRangeStep

[nalimilan]

As discussed at https://github.com/JuliaLang/julia/pull/25402#issuecomment-355628275 and https://github.com/JuliaLang/julia/issues/20402#issuecomment-354769392, we should find better names for traits defined in traits.jl, since they are the only ones whose names start with Type. Here's a list with suggestions:

• TypeOrder (possible choices: HasOrder/Unordered) -> OrderStyle (Ordered/Unordered), HasOrder (Ordered/Unordered) or IsOrdered (Ordered/Unordered)
• TypeArithmetic (ArithmeticRounds/ArithmeticOverflows/ArithmeticUnknown) -> ArithmeticStyle or ArithmeticAccuracy (BTW, it would make sense to add ArithmeticExact and divide overflow into ArithmeticWrapsAround/ArithmeticSaturates.)
• TypeRangeStep (RangeStepRegular/RangeStepIrregular) -> RangeStep or RangeStepStyle

[StefanKarpinski]

I like using the Style suffix consistently:

• OrderStyle: Ordered, Unordered and in the future possibly PartiallyOrdered.
• ArithmeticStyle: ArithmeticRounds, ArithmeticWraps (I assume that's what ArithmeticOverflows means), ArithmeticUnknown, and future possibly ArithmeticSaturates.
• RangeStepStyle: RangeStepRegular, RangeStepIrregular.

[StefanKarpinski]

Triage likes the -Style suffix names. Not all traits need to end in style, but it seems like a good pattern.