If stdio is dead due to EPIPE, there's no great reason to spew a stack dump.
This will still write an error to stderr if stdout dies. This might be
undesirable, but changing that should be considered separately.
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
