Cleaning CPP builds

[coltfred]

The C++ headers and build files are not easy to cleanup today. For example, if you want to cleanup the cmake files you run something like this from the cpp dir.

rm -rf Makefile CMakeCache.txt CMakeFiles cmake_install.cmake rust_swig_* ironoxide_regen_*

If you want to cleanup the dynamic library:
cargo clean or rm -rf target/ironoxide_shared.* from the root.

If you want to cleanup the headers you run rm -rf cpp/generated

Needless to say, this is a mess. We should figure out which system we want to use to clean things up and add commands to do it.

[ernieturner]

Another option would be to put the generated C/C++ into the target directory, like how Java works.