Attempt to simplify ZeroCopyFrom

[sffc]

I think we should be able to move the 'b in ZeroCopyFrom up to the trait and then remove the Yokeable bound. CC @Manishearth

If we add FromVarULE in #1180, we may be able to merge that trait and ZeroCopyFrom.

[Manishearth]

Yeah if we implement ZCF on the Output types instead of the input types stuff will be much nicer, and we'll hit fewer of those compiler bugs.

[Manishearth]

I think then it needs to be

trait ZCF<'a, T>  {
   fn zcf(&'a T) -> Self
}
impl ZCF<'a, str> on Cow<'a, str> {
   fn zcf(&'a str) -> Self
}

This is definitely nicer IMO

[Manishearth]

Oh, we have duplicate issues; oops

#1255