`[try_]pin_init!` should check that the initialized struct is `!Unpin`

[BennoLossin]

[try_]pin_init! should check that the initialized struct is !Unpin:

• if it is Unpin, [try_]init! can be used instead (since all structurally pinned values must then also be Unpin)
• it can catch people forgetting PhantomPinned in their structs