add an "expand" function to ninja_syntax

Implements basic variable expansion for use in configure.py.
add a --bootstrap mode for configure.py

Instead of bootstrapping through a separate script, instead make
configure.py able to either generate a build.ninja *or* just execute
all the computed commands to build a ninja binary.
drop bootstrap.py

We now use configure.py --bootstrap.  Direct users of this script to
use the other one.