revert #1470; fix #1493
add Exception#backtrace test
fix crash in 2nd call to vsnprintf
add verbose_backtrace
print singleton class name in verbose backtrace
support multiple filename in irep
use uint16_t for line type
reduce node size
rename debug function name
support c++ in mruby/debug.h
better line type selector
declare variable outside for
don't use RArray in filename table generating
improve checkings in debug.c
fix line number after first file
remove Exception#verbose_backtrace; #1495