Add new Debug Flag -DZ

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Please take a few moments to verify that your commits fulfill the following:

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• Add a runtime debug flag -DZ to zero out heap memory after being freed by gc. Motivated by https://github.com/bgamari/ghc-debug/blob/master/call-for-participation.md