stop conv from mutating input

[vincentcp]

Closes #67

[dlfivefifty]

I would suggest a two stage version: conv! which mutates and conv which calls conv!.

Also, add tests please

[codecov]

Codecov Report

Merging #89 into master will decrease coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
- Coverage   80.83%   80.76%   -0.07%     
==========================================
  Files          12       12              
  Lines        1247     1248       +1     
==========================================
  Hits         1008     1008              
- Misses        239      240       +1

Impacted Files	Coverage Δ
src/fftBigFloat.jl	82.03% <100%> (+0.14%)	⬆️
src/specialfunctions.jl	53.4% <0%> (-0.36%)	⬇️

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