Implement heap snapsnot memory tracking for Event, EventTarget

[jasnell]

Expands heap tracking memory tracking for handful of API types

Includes the context global in the heap tracking trace. originally had some extra stuff in here for special-case handling of the context global in order for it to appear in the snapshot a certain way but it added way more complexity than it was worth. The non-special case handling is sufficient.

Refines a few bits and pieces in the memory tracker def

[hoodmane]

I don't fully understand the changes to memory.h. Everything else looks reasonable to me. One question: what is the strategy to keep these visitForMemoryInfo implementations up to date as the code changes? I guess it's a similar business to having to remember to tell the garbage collector about fields you add.

[jasnell]

I guess it's a similar business to having to remember to tell the garbage collector about fields you add.

Yep, we just have to be disciplined to keep it up to date. Fortunately there's less impact here if we get this wrong than with visitForGc()

[jasnell]

There are still a few things I want to look at here, and I might end up pulling the further simplifications made to memory.h in one of the other prs into this... going to move this back to draft so it doesn't get merged before then