update LIBJ_DEBUG_GC
rename LIBJ_DEBUG_GC to LIBJ_GC_COLLECT
fix a bug of json::stringify
introduce LIBJ_GC_DELETE
modify tests of LIBJ_GC_DELETE
update leak-check.rb
make detail::gcollect more simple if LIBJ_DEBUG is not defined
introduce LIBJ_GC_COLLECT_PER
fix a build error
print 'start' and 'finish' of full gc