Reintroduce seqan3::literals

[marehr]

Description

See old discussion why we abandoned it seqan/seqan3#152.

Acceptance Criteria

• test 1
• test 2
• test 3

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

[marehr]

2021-02-08 Core Meeting

We miss seqan3::literals. We think the original issue was that seqan3::literals (in plural because the std names it also plural) wasn't defined as an inline namespace. S.t. you needed to "import" seqan namespaces twice:

in the old times:

using namespace seqan3;
using namespace seqan3::literal;

"ACGT"_dna4

Now:

using namespace seqan3::literals;

"ACGT"_dna4

or

using namespace seqan3;

"ACGT"_dna4

The main problem is that we discourage to use using namespace seqan3;, but if you want to use a literal, you need to import it with a weird syntax using seqan3::operator""_aa10li;. Now you can use using namespace seqan3::literals;

Tests should still import it explicitly instead of using namespace seqan3::literals;. Snippets should use using namespace seqan3::literals;.

We sometimes have

 * \attention
 * All seqan3 literals are in the namespace seqan3!

these should be removed without replacement.

namespace seqan3
{
inline namespace literals
{
auto operator""_aa10li(...);
} // namespace literals
} // namespace seqan3