Improve support for complex intervals

[OlivierHnt]

The goal of this PR is to address #41 and and #295.

TODO: sqrt, ^, and inverse trigonometric/hyperbolic functions.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 21.42857% with 22 lines in your changes are missing coverage. Please review.

Project coverage is 82.25%. Comparing base (c55dd23) to head (30cee9c).

Files	Patch %	Lines
src/intervals/arithmetic/power.jl	6.25%	15 Missing ⚠️
src/intervals/arithmetic/trigonometric.jl	20.00%	4 Missing ⚠️
src/intervals/arithmetic/hyperbolic.jl	0.00%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #636      +/-   ##
==========================================
- Coverage   82.98%   82.25%   -0.74%     
==========================================
  Files          26       26              
  Lines        2181     2203      +22     
==========================================
+ Hits         1810     1812       +2     
- Misses        371      391      +20     

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[OlivierHnt]

I think this PR should be merged ASAP, even though it does not provide support for sqrt, ^, and inverse trigonometric/hyperbolic functions.
It can always be done in a next PR.