Add support for the event section

[aheejin]

This adds support for the event section and fixes a few existing typos.

[aheejin]

Currently the LLVM CL does not implement this, but added this entry for symmetry. (WASM_COMDAT_GLOBAL is not implemented either). And I guess COMDATA was a typo for COMDAT, because in LLVM they are named as COMDAT).

[sunfishcode]

Can you explain somewhere in the doc what an "event" is?

[xtuc]

IMO "event" is confusing, but I can't find a better name for it. Maybe "control-flow event"?

[rossberg]

I agree that "event" is not the right term. The usual name for this is either an effect, or more specifically, an exception. I think we can just stick to "exception".

[aheejin]

I think we reviewed and agreed on the term 'event' in this PR when the second proposal was written in the beginning of this year. It is meant to include possible future constructs that may be supported later. The current LLVM and V8 implementation uses this term too.

The 'attribute' field of an event entry in the event section is meant to denote which kind of event it is. Currently the only possible attribute is EXCEPTION, but other values can be added later.