Implement `complex` for dual arguments

[sethaxen]

This PR implements the following methods:

• complex(::Dual, ::Real) -> Dual{<:Complex}
• complex(::Real, ::Dual) -> Dual{<:Complex}
• complex(::Dual, ::Dual) -> Dual{<:Complex}
• complex(::Type{Dual{T}})

This allows Dual to be used for automatically differentiating not just the complex functions for which rules are defined here but also the construction of those complex numbers.

[codecov-commenter]

Codecov Report

Merging #90 (f5a6d29) into master (7848bf4) will increase coverage by 1.36%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
+ Coverage   44.67%   46.03%   +1.36%     
==========================================
  Files           1        1              
  Lines         197      202       +5     
==========================================
+ Hits           88       93       +5     
  Misses        109      109              

Impacted Files	Coverage Δ
src/dual.jl	46.03% <100.00%> (+1.36%)	:arrow_up:

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[dlfivefifty]

Looks fine