The main goal here is to remove the need for a C API to the code in
`mesh_boolean_convert.cc`. This is achieved by moving `MOD_boolean.c`
to C++ and making the necessary changes for it to compile. On top of
that there are some other slight simplifications possible to the
direct mesh boolean code: it doesn't need to copy the material
remaps, and the modifier code can use some other C++ types directly.
