Add PoolArray::to_vec(), improve doc around read()/write()

[Bromeon]

Closes #806.

I noticed that the documentation around the read() and write() methods was a bit lacking, and tried to make it a bit easier to understand. Also moved trait impls after the inherent impl.

I'm not 100% sure that we really need to_vec(). It's nice as a convenience, but it may cause people to do things like

pool_array.to_vec().map(...).collect()
// or
for elem in pool_array.to_vec()

instead of going via read(). After all, read().to_vec() is not much longer.
On the other hand, we already have PoolArray::from_vec(), so it's somewhat symmetric.

I'd be happy for some input on that.

[Bromeon]

bors r+

[bors]

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