Add Layer::into_raw

[madsmtm]

Allow consuming the Layer, which allows avoiding an unnecessary retain/release in the fairly common case where the user was going to retain anyhow.

Draft because it builds upon #19.

[MarijnS95]

I wish as_ptr() was named as_raw() in that case (and I have to do a similar refactor on the NDK 😩).

[madsmtm]

Hmm, maybe we call this into_ptr then instead? Idk., I don't care much for the naming, so I will leave that decision to you, but I will note that Box uses as_ptr and into_raw...

[madsmtm]

I'm gonna go with into_raw, as I think it communicates more clearly "give up ownership of the pointer", though I will differ from std by returning NonNull instead, will do so in a later PR.

[MarijnS95]

Idk either, it's already inconsistent in std anyway. Maybe I'll use this standard as a guide for the NDK though, which is currently incredibly mixed anyway.