Unfortunately this is not pure C++20 :(

TODO: Write unit test for connection_line_buffer using loopback client.
Warning: Do not rely on connection_line_buffer::conn_data_in_cb until the unit test case proves its correctness (Nice looking solution but kinda smells like an off-by-one)

`get_send_data()` doesn't work yet as I haven't found a C++ style way to safely cast const std::string& to std::string in line 24 of loopback_connection.cpp

Also includes several const correctnet fixes and type simplifications

Workaround for an issue in GCC w.r.t. -Weffc++ on operator= …