Trying to add watchpoint function to bdb and pdb. It's goal is to do as gdb for three different watch: watch, rwatch and a watch. watch: when expr is written by program and value has changed, it will break rwatch: when expr is read by program, it will break awatch: when expr is read or write by program, it will break
watch and awatch will detect value change and breakout, rwatch using dis module to get the file bytecode, with the lineno it can detect current line is load (read) or not.
