Express reshape to a certain dimensionality as conversion instead?

[mbauman]

We've slowly been converging on the final semantics of our post-#4774 arrays. We've nailed down equality (dimensions are significant in terms of equality), and our indexing behaviors post-#14770 have a nice set of rules about how trailing singletons behave with respect to trailing dimensions:

• You're allowed to specify fewer indices than there are dimensions so long as all omitted dimensions are 1.
• You're allowed to specify more indices than there are dimensions so long as all extra indices are 1.

I think this could dovetail quite nicely with conversion — would could allow convert(AbstractArray{<:Any, N}, A) to simply add or remove trailing singleton dimensions. In fact, I think that's a slightly better spelling of this behavior than reshape(A, Val(N)).

[timholy]

👍 I think formerly one would have had to manually allocate a TypeVar for this syntax, but with the new type system it's much prettier.