Add mapAccumM, forAccumM to Data.Traversable

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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][notes] and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a [Note][notes] and cross-reference it from the relevant places.
• add a testcase to the testsuite. The base library does not have the tests for Data.Traversable yet. I can add them later to this MR.
• Implementation of the Core Libraries Proposal