[FIX] fix internal compiler error on gcc-7

[marehr]

Changes in #2427 triggered an ICE (in Fact the same one that @MitraDarja had in #2337).

This is an attempt to fix this ICE once and for all by using an "easier" way of using is_valid_char at compile time.

Also part of seqan/product_backlog#124, since it throws away some meta-library functionality.

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[codecov]

Codecov Report

Merging #2430 (9bba93f) into master (08bab5d) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2430      +/-   ##
==========================================
- Coverage   98.24%   98.24%   -0.01%     
==========================================
  Files         267      267              
  Lines       10917    10912       -5     
==========================================
- Hits        10725    10720       -5     
  Misses        192      192              

Impacted Files	Coverage Δ
...ude/seqan3/alphabet/composite/alphabet_variant.hpp	100.00% <100.00%> (ø)

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[Irallia]

I have some ideas and questions about the tests.

[Irallia]

Thanks for the explanation.