🎨 Improvements for bitset

[elbeno]

Problem:

• bitset construction from a string_view uses substr which may throw, and it's a really niche use case anyway.
• There's no need for for_each and transform to be friends of bitset since the member functions are public.
• It's awkward that bitset<16> and bitset<16u> are different types.

Solution:

• Remove bitset's string_view constructor.
• Remove the friend declarations.
• Fix the argument of bitset to a std::size_t in the cases it's not an enum.