[swift-evolution] [Draft] Abolish IUO type

[Anton Zhilin]

+1 for the idea. It will extract compiler magic for IUO types into a variable declaration attribute.
-1 for current wording/details.

1. We have to add @implicitly_unwrapped attribute to the proposal. I hate implied "attributes", available only to compiler. The wording needs to be reformulated in terms of this attribute
2. We need to encourage using the attribute instead of `!`. In order to do this, I suggest finding a concise name for it. Like @IUO, but possibly more readable
3. In the future, `!` can be removed entirely and leave only the attribute. But not for now

- Anton