`FrozenVec::into_vec` should not require `StableDeref`

[LHolten]

FrozenVec::into_vec takes ownership of self. This means there can not be any references to elements in the FrozenVec.

Removing the StableDeref bound will make it possible to use FrozenVec to build vectors of elements that are not StableDeref.

[Manishearth]

This makes sense, we should do it.