Simplify OwnedFace; remove usage of Pin

[dhardy]

As I understand it, the raison d'etre of Pin is to allow returning a type to the user which cannot be moved. Without a &mut reference or direct ownership of the (unboxed) value this isn't possible anyway. See here.

Further, we don't actually care about SelfRefVecFace being moved, we only care about the data (which is itself heap-allocated) being moved.

I left SelfRefVecFace boxed only because the face value is large enough that it probably shouldn't be copied around too much, but it's not required for safety.

This PR simplifies things significantly without compromising safety or breaking the API.

[dhardy]

On further thoughts, it doesn't really make sense to box SelfRefVecFace then store this in an Arc (FontArc). So maybe we should just rename SelfRefVecFace → OwnedFace and stick #[inline] on the ctor? I'll go ahead if you agree, but it does mean if the result is not immediately stored inside a Box/Rc/Arc that some unnecessary copying will happen.

[lunabunn]

On further thoughts, it doesn't really make sense to box SelfRefVecFace then store this in an Arc (FontArc). So maybe we should just rename SelfRefVecFace → OwnedFace and stick #[inline] on the ctor? I'll go ahead if you agree, but it does mean if the result is not immediately stored inside a Box/Rc/Arc that some unnecessary copying will happen.

While I realize that this PR is currently stale, just for the record: this (removing the Box) would be unsound under both SB and TB. See #14.