Sketching out more efficient variadics

[anandabits (Matthew Johnson)]

@jrose

I don't want to discuss the syntax for this , just whether people think it's reasonable to allow other types to be used for variadic parameters besides Array. (I only briefly skimmed the old discussions so I should probably read those again.)

I am a huge fan of this idea (and even started a thread on it once Enhanced Variadic Parameters), thanks for reviving it!

@Joe_Groff

We could address the "how do we stack promote more arrays" problem more generally, without specifically targeting variadics, by having a "copy on escape" model for all value type buffers

It would be really cool if we had a consistent and straightforward way to reason about when stack promotion of arrays would happen. Are there similar changes we could make to facilitate similar guarantees around arrays that are returned from functions (at least for inlinable functions)?