[MISC] Update writable_alphabet concept

[MitraDarja]

Resolves seqan/product_backlog#278

[MitraDarja]

@marehr

Sorry, but I can not figure out the error myself.
I have tried deleting requires SEQAN3_IS_CONSTEXPR(seqan3::assign_char_to(alphabet_char_t<t>{}, std::remove_reference_t<t>{}));, but the ice remained. Adding requires SEQAN3_IS_CONSTEXPR(seqan3::char_is_valid_for<t>(alphabet_char_t<t>{})); again, removes the ice.

My test script is:

#include <seqan3/alphabet/all.hpp>

int main()
{
    seqan3::char_is_valid_for<seqan3::gapped<seqan3::dna4>>('a');
    return 0;
}

I checked if seqan3::char_is_valid_forseqan3::dna4('a') and seqan3::char_is_valid_forseqan3::gap('-') work and they don't lead to an ice. I also tried using dna5, but there the error remained.

[marehr]

Let's see whether my fix fixes it :)

[codecov]

Codecov Report

Merging #2337 (d159be3) into master (44ba173) will decrease coverage by 0.00%.
The diff coverage is 61.53%.

@@            Coverage Diff             @@
##           master    #2337      +/-   ##
==========================================
- Coverage   98.21%   98.21%   -0.01%     
==========================================
  Files         263      264       +1     
  Lines       10772    10784      +12     
==========================================
+ Hits        10580    10591      +11     
- Misses        192      193       +1     

Impacted Files	Coverage Δ
...ude/seqan3/alphabet/composite/alphabet_variant.hpp	100.00% <ø> (ø)
include/seqan3/alphabet/concept.hpp	100.00% <ø> (ø)
include/seqan3/io/alignment_file/format_sam.hpp	96.33% <0.00%> (-1.80%)	⬇️
...de/seqan3/io/alignment_file/sam_tag_dictionary.hpp	100.00% <ø> (ø)
...nclude/seqan3/core/configuration/configuration.hpp	100.00% <100.00%> (ø)
include/seqan3/core/detail/debug_stream_byte.hpp	100.00% <100.00%> (ø)
...n3/search/dream_index/interleaved_bloom_filter.hpp	100.00% <100.00%> (ø)
include/seqan3/core/detail/debug_stream_type.hpp	76.92% <0.00%> (+15.38%)	⬆️

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[simonsasse]

Looks good to me 🥳