Pre SIP: Replace non-sensical @unchecked annotations

[morgen-peschke]

@odersky

We use implicit conversions (user- or system-defined) all the time, and yet nobody so far has proposed “being convertible” should be reified into a type.

@Ichoran

That’s because the reification is done by a given Conversion being in scope. I like that much better than the old completely-non-explicit way of doing things.

Aside from Conversion doing exactly this, it’s been suggested at least once. Treating things as type is unreasonably useful