Add unsafeToArray to IArray

[mpilquist]

This came up when integrating IArray with fs2, where I needed access to copyToArray. Rather than add copyToArray as an extension method to IArray, this PR provides an escape hatch to access the underlying mutable array without needing an explicit asInstanceOf.

[smarter]

Is the change to the dotty-cps-async submodule in this PR intentional?

[mpilquist]

@smarter No, thanks for spotting, fixed

[sjrd]

I don't think this is a good idea. IMO if you "need" to cast IArray into Array, you should have to use asInstanceOf, so that you realize how dangerous it is.

IMO this direction of cast is significantly more hazardous than casting mutable array into an immutable one. In the latter situation, you already have ownership of the mutability of the array. You are choosing to give, most likely temporarily, a read only view of the array to someone who only needs to read. It's your responsibility not to mutate the array during that time, but that's more or less OK because you are the owner of the mutability.

In the scenario that this PR addresses, you are most definitely not owning the mutability of the array. You are granting yourself that capability although it was not yours to grant. This is qualitatively more unsafe, IMO.

I don't want to "judge" your use case. It may very well be that it's very important not to copy, and I trust that you know what you're doing, with great care. But that's what asInstanceOf is for: an escape hatch for great care needs.

[mpilquist]

Closing in favor of #10743