When determining the old path, get the existing value in any scope,
not just the set scope. Also only complain about absolute paths:
relative paths are expected to be invalid sometimes.
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
