Add missing complex tests and rules

[sethaxen]

This PR fixes #215 by adding missing complex tests and where necessary generalizing rules and adding missing rules. New rules are

• adjoint(::Number)
• real(::Number)
• imag(::Complex)
• Complex(::Number)
• Complex(::Number, ::Number)
• hypot(::Complex)
• hypot(::T, ::T) where T<:Union{Real,Complex}
• exp(::Number)

It also simplifies the rules for abs, angle, and sign with shared utility functions.

I believe as of this PR, all scalar functions in Base for which we have complex rules should be tested on complex inputs.

[sethaxen]

The one failure on Julia 1.4.2 is odd. muladd(x,y,z)'s rules are defined using @scalar_rule, which literally implements the primal for frule as muladd(x,y,z), yet somehow the equality check against muladd(x,y,z) fails for complex inputs (the values are still approximately equal). JuliaDiff/ChainRulesTestUtils.jl#46 relaxes the equality check, but that still doesn't explain why it fails here. I cannot reproduce this locally.

[willtebbutt]

Hmm yeah, that is very strange... checking for approximate equality is fine though imho.

[oxinabox]

should these all be AbstractComplex ?

[sethaxen]

I don't think there is an AbstractComplex in base.

[sethaxen]

Okay this should be ready for review.

@MasonProtter I ended up simplifying the rules for abs, angle, and sign with shared utility functions. They pass the same test suite, and locally their runtime seem to be the same as the specialized versions you implemented, but it'd be nice if you could double check.

[MasonProtter]

Looks good to me!

[willtebbutt]

Minor style thing. Otherwise LGTM