Rather than storing short and long options separately, using
a complicated set of invariants, store them in a single string
and use an explicit type complete_option_type_t to track how they
are interpreted.
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.
In a few places, we need to add a prefix to completions that
replace the token. This change factors that logic into its
own function prepend_token_prefix.
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
