Avoid use of unsafe for Vec storage

[philipc]

This change is intended to be transparent to users. However, I did have to add a T: Clone bound for implementations of ArrayLike.

r? @nbdd0121

[philipc]

I don't know how to fix the 1.42 build, so might have to leave this for later.

[philipc]

This isn't going to land in its current state. The basic idea is to add a new trait for Vec operations to allow direct use of Vec, instead of needing to wrap it in ArrayVec. This new trait is implemented for both Vec and ArrayVec. Except it needs to be split in two traits because we don't expose ArrayVec in the public API. This PR currently renames traits, but I think it would have been better to leave ArrayLike as is and add a new VecLike trait.

Additionally, to enable deriving of traits like Clone, we then we need to put the Vec/ArrayVec in a newtype and manually implement those traits, but this PR didn't do that yet.