Remove superfluous std::string copy operations

[NaN-git]

Motivation

Nix uses std::string in many places. Often those are copied around, although this is not necessary because std::string_view can be used instead.

Also for most regex related code std::string_view is sufficent, too.

Context

Nix has leftovers from code, which used C-strings in the past. Some of these leftovers are fixed.

C++20 adds the view() member function to std::basic_ostringstream. Sadly, the libc++ library shipped with clang-16 does not implement this and clang-17 would be required. Thus a workaround is implemented to get a std::string_view from std::ostringstream.

The PR reduces closure size by ~50KB. When evaluating nixpkgs I couldn't measure a significant performance difference.

Priorities and Process

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[roberth]

Thank you!