Restructure the getters for event fields to
be more targeted at particular event types.
Update all callers, and replace all direct
event struct access with getters.
As a side-effect, this drops some unused getters.
-Wint-conversion is important because it checks casts from ints to
pointers.
-Wdiscarded-qualifiers is important to catch cases where we don't
strings when we should.
