Simplify EventLog interface -- remove supported for nested

sequences (which nobody used) and streamline the API, adding
documentation in preparation for inclusion in the SDK.

Gut and deprecate EventLogTags, which unfortunately was put
into the public SDK (an oversight).  Include the functionality
in EventLog proper, in a simpler and easier to use manner.

This change doesn't actually un-@hide anything, but it does
change it to @pending.