Move some generally useful Python functions from the various tools under tools into its own library, tentatively tools/py_common, such as packed integer decoding, checkpoint cookie decoding, page/block data structures, and the like.

There are already some functions in tools/wt_tools_common.py, which should be moved under tools/py_common as well. Given the scope of refactoring, placing common functions into a directory with multiple files is preferable to moving them all into the same file.