Introduces a new template moved_ref which is like an rvalue reference.
This allows passing around objects while being explicit that the
receiver may acquire ownership. This will help reduce some allocations.
This change eliminates global variables like stdout_buffer. Instead we wrap up
the IO information into a new struct io_streams_t, and thread that through
every builtin. This makes the intent clearer, gives us a place to hang new IO
data, and eliminates the ugly global state management like builtin_push_io.
This change moves source files into a src/ directory,
and puts object files into an obj/ directory. The Makefile
and xcode project are updated accordingly.
Fixes#1866
