Prior to this fix, read_ni would use parse_util_detect_errors
to lint the script to run, and then parser_t::eval() to execute it.
Both functions would parse the script into a parse tree. This allows
us to re-use the parse tree, improving perfomance.
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
