IME Events are now completely variable sized.

[StayTalm]

IME events now have no cap. This handles a new event that comes from native Unity that has a variable sized IME event. Do not merge until new event is available.

-Updated IME test to still work.

[StayTalm]

@renedamm should be good for re-review, moved to using Unity's unsafe & native array tools, improved the docs.

Added 1 test, and was already using the existing IME event test to verify that the composition events get received properly.

[unity-cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[jimon]

I'm recreating this work in #1628