`FromZeros::insert_vec_zeroed` both panics and returns `Result`

Fallible functions that also panic is a general API smell. [`insert_vec_zeroed`](https://docs.rs/zerocopy/0.8.14/zerocopy/trait.FromZeros.html#method.insert_vec_zeroed) mirrors [`std`'s](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.insert) panicking on index, but doesn't mirror the panicking allocation.

In the next breaking version, I propose that `insert_vec_zeroed` be made fully fallible by returning `Err` if `position > v.len()`.

Relevant: #1653