panic! at the polymorphism failure

[syvb]

Always panic! in the from_datum implementation for AnyElement/AnyArray instead of only panicking when debug assertions are enabled.

These functions should never be called anyways, so there's no real performance benefit by only panicking in debug mode.

[workingjubilee]

I'm surprised all our tests pass.

Can you update the documentation appropriately so people know they should never call these?

I'm not sure exactly which places (on struct AnyElement itself, maybe?) since the documentation on a FromDatum impl isn't very visible, but whereever seems appropriate is fine by me.

[syvb]

I added some documentation for Any{Element,Array}. The documentation for them is very similar, but I figured it wasn't worth it to split the documentation into a separate file to be reused by both.

In the future it might be worth considering using a macro to generate the any* modules since they're almost identical, and it would allow us to support the other polymorphic types easily.

[workingjubilee]

Thank you! Yeah, I think this is good then.

[workingjubilee]

It's true that we should basically never be using from_datum for this and should be using from_polymorphic_datum always.