generate core files in a directory

[harendra-kumar]

1. Currently with the dump-core option we place the core files in the same directory as the source file. It pollutes the source directory. We can instead create a directory with name of the source file and place all cores in that dir.

2. We can specify the core-dir using a command line flag as well. That way we can dump cores from different runs into different dirs and then run a script to automatically diff the runs.