allow verbose tests

This can be helpful for debugging the process mysteriously
exiting during the tests, for example.
Make tests compile quicker

This more than fixes the compile-time-slowdown of the previous commit.
It adds compile flags TEST_CONFIG_COMPILE_QUICKLY
and TEST_CONFIG_VERBOSE near the top of test_header.hpp
that can be tweaked while tracking down a test failure
if the faster-compiling behavior isn't useful enough.
clean up already-fixed TODO comments
compile-time-bignum: interpret numeric constants more 'base'-independently
add a units test
Remove some test warnings

(without hurting compile time much or affecting the code)
add conversion of compile-time bignum to runtime bignums of any base
make compile-time-bignum convertible to chars/string