Define real, imag, conj, angle, and abs2 for Vec{N,<:Real}

[favba]

Addresses #126

[favba]

Hi, @KristofferC
This a quite simple PR that implements complex number functions (real,imag,angle,abs2) for the supported types by SIMD.jl

It is useful for calling with Vecs generic definitions of functions that should work for both complex and Real numbers. That is indeed how I encountered this issue (I used conj and real functions).

I didn't implement angle(::Vec{N,Integer} as that would involve a different output type, and I thought the conversion might be better handled explicitly by the programmer.

This PR is pretty much complete now.

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All modified and coverable lines are covered by tests ✅

Project coverage is 88.86%. Comparing base (c332a03) to head (1110296).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   88.77%   88.86%   +0.09%     
==========================================
  Files           5        5              
  Lines         561      566       +5     
==========================================
+ Hits          498      503       +5     
  Misses         63       63              

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[favba]

The failures in Julia nightly are unrelated to the PR changes