util: extract all date-related utils in utils/dateutil module
With this commit, util.py lose 262 lines
Note for extensions author, if this commit breaks your extension, you can pull
the step-by-step split here to help you more easily pinpoint the renaming that
broke your extension:
hg pull https://bitbucket.org/octobus/mercurial-devel/ -r
ac1f6453010d
Differential Revision: https://phab.mercurial-scm.org/D2282