Use the newest Layout::array size checks for vec-alloc

[fpdotmonkey]

Layout::array was updated in rust-lang/rust@a32305a8 to check that array sizes fit in isize::MAX, so I've propogated the change to vec-alloc here.

[fpdotmonkey]

I'm curious if the expect could be moved into the else block. This would mean you wouldn't get the nice error message for the case of [T; 1] where size_of::<T>() > isize::MAX, but is that actually possible to make?

[scottmcm]

Correct, no allocation or type in rust can have size greater than isize::MAX -- that's how the alloc::Layout type can use the unchecked constructor from size_of+align_of.