Define interfaces by tests

[mofeing]

This PR defines a series of functions for testing whether a type fulfills some interface. This is the first step on replacing our current Quantum & Ansatz classes, that act as concrete mixins for other classes, for Pluggable and Ansatz abstract interfaces. By adding these tests, we test for API breakage while we refactor internals.

This should allow us to fix the type hierarchy serialization problems we suffer from Julia's single-inheritance enforcement, while also avoiding the diamond problem and schizofrenia-like problems of multiple-inheritance OOP.

[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.24%. Comparing base (9f298ae) to head (ee82904).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Circuit.jl	93.75%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #312       +/-   ##
===========================================
+ Coverage   64.76%   86.24%   +21.47%     
===========================================
  Files          36       34        -2     
  Lines        2401     2428       +27     
===========================================
+ Hits         1555     2094      +539     
+ Misses        846      334      -512     

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