Add some more OOM-handling Box<[T]> constructor variants

[fitzgen]

Part of #12069

[alexcrichton]

Another possible idea:

• One function to return fn(usize) -> Result<Box<[MaybeUninit<T>]>>
• Another function fn(Iterator<T>, Box<[MaybeUninit<T>]>) -> Result<Box<[T]>>
• Another function for a fallible iterator
• Helper functions combining these together

[fitzgen]

Another possible idea:

• One function to return fn(usize) -> Result<Box<[MaybeUninit<T>]>>

• Another function fn(Iterator<T>, Box<[MaybeUninit<T>]>) -> Result<Box<[T]>>

• Another function for a fallible iterator

• Helper functions combining these together

This is sort of what we have, except we don't expose the second function publicly, just as an implementation detail.

What these proposed functions don't collectively cover, however, is when you have an unknown-length iterator, unless I am misunderstanding.

[fitzgen]

Okay the boxed slice helpers are now implemented on top of our OOM-handling Vec and this PR now depends on #12452 (and therefore also #12448)