Refactor codebase to use 'cpp11'

[pachadotdev]

Dear 'httpuv' maintainers:

I hope you are doing well.

I took all the feedback from the previous thread, and decided to try with cpp11.

I registed functions of the form my_fun_() on C++ side and roxygenised wrappers of the form my_fun() on R side, the changes go smoothly.

Similar to 'later' (PR 269), but without a side 'httpuvtest' package as there is no API here, I looped multiple devtools::check() calls for different pairs such as "Clang + CXX17/20/23" and "GCC + CXX17/20/23". In addition, same for 'later', I use https://github.com/wch/r-source to run make build-r-devel && make check-devel to build R-devel in a sub-directory and there install devtools and all the dependencies to run an extra devtools::check().

I know this testing may read a circular but Clang usually shows things that GCC accepts silently.

Please let me know about this PR. I tried to edit as little as possible.

Be safe and well,
MVS

[pachadotdev]

@estebanlp here's the other, same ABI thing