Work toward compiling tinyjs.

This includes out_sample12.py as a target code output from sample12.py.

I realized I'd like virtually all the work to happen at runtime, since js is a
very introspective language.  I also decided that I'll switch from a single run
command to a set of commands that work together with a stack-like interface to
simplify nested indents and src-to-code line-by-line tracking for debug output.