Make a few constant globals actually const.

This means the .data and .bss sections are empty, a good thing
in a shared library (see #98).
Combine the functionality of jq and jq_test

One binary is much simpler. ./jq --run-tests now runs the tests.
WIP
Remove JQ_DEBUG #define and jq_test binary, simplifying build.

The debugging features previously available via JQ_DEBUG are now
command-line options.