cxx20: return-type-requirement is not a type-constraint

[marehr]

Description

The following expression

template <typename t>
concept semialphabet = requires (t v)
{
    { seqan3::to_rank(v) } -> seqan3::alphabet_rank_t<t>;
}

is invalid starting with C++20.

In C++20 you would use the following expression :

template <typename t>
concept semialphabet = requires (t v)
{
    { seqan3::to_rank(v) } -> std::convertible_to<seqan3::alphabet_rank_t<t>>;
};

(The return type needs a concept requirement)

The problem is that gcc 7 and 8 does not support the C++20 syntax.

A compromise that works for every compiler version is

template <typename t>
concept semialphabet = requires (t v)
{
    requires std::convertible_to<delctype(seqan3::to_rank(v)), seqan3::alphabet_rank_t<t>>;
};

Acceptance Criteria

• Replaced most of these occurrences by the compromise solution

Tasks

• task 1
• task 2
• task 3

Definition of Done

• Implementation and design approved
• Unit tests pass
• Test coverage = 100%
• Microbenchmarks added and/or affected microbenchmarks < 5% performance drop
• API documentation added
• Tutorial/teaching material added
• Test suite compiles in less than 30 seconds (on travis)
• Changelog entry added