Open OperatorEnum to complex numbers

[MilesCranmer]

This allows DynamicExpressions.jl to work on complex numbers using the (faster) OperatorEnum type. This also adds tests for ComplexF32 and ComplexF64.

[coveralls]

Pull Request Test Coverage Report for Build 4432086150

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 90.775%

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Totals
Change from base Build 3558750421:	0.4%
Covered Lines:	925
Relevant Lines:	1019

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💛 - Coveralls