In the compiler class, replace the backend and is_native methods
by host and target ones.
In the toplevel class, get rid of the is_native method which is useless
at the moment.
When a toplevel test consists in more than one file, it is necessary to
compile the auxiliary files so that they can then be loaded into the toplevel.
This method stores for each toplevel the compiler to use to do this.
Nicolás Ojeda Bär
Sébastien Hinderer