v.0.6.15

This is a maintenance release.

A fix for CMake script for case when RESTINIO_FIND_DEPS=ON and RESTINIO_FMT_HEADER_ONLY=OFF (#159).

Usage of std::aligned_storage_t in implementation of writable_item_t has been removed (it's deprecated in C++23).

In some places std::launder is used (if available) to avoid UB accessing a pointer after placement new.

NOTE: restinio-0.6.14-full.* archives contain asio-1-21-1, fmt-8.1.1 and catch-2.13.9. Versions of 3rd party libraries in restinio/third_party weren't upgraded.