Enhance test_ambiguities

[lgoettgens]

As written in the removed comment, test_ambiguities does not allow non-singleton callable in exclude. This is working now.
I furthermore changed the behavior for singleton callables to be the same. And I introduced constructors for exclude.
Unfortunately, one can only exclude the constructor of type MyType and all callables of MyType simultaneously, or none of both.

For my convenience, this already includes the changes of #143.

This change is probably breaking, semi-documented behavior of exclude for test_ambiguities. I thus bumped the version.

[codecov]

Codecov Report

Merging #144 (a38c5e1) into master (c7eb4d4) will decrease coverage by 0.08%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
- Coverage   77.14%   77.07%   -0.08%     
==========================================
  Files          11       11              
  Lines         722      724       +2     
==========================================
+ Hits          557      558       +1     
- Misses        165      166       +1     

Flag	Coverage Δ
unittests	77.07% <83.33%> (-0.08%)	⬇️

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Impacted Files	Coverage Δ
src/ambiguities.jl	89.60% <83.33%> (-0.65%)	⬇️

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[fingolfin]

Needs a rebase