* Introduce a mechanism into Sema for emitting a compile error when an
   integer is too big and we need it to fit into a usize.
 * Add `@intCast` where necessary
 * link/MachO: fix an unnecessary allocation when all that was happening
   was appending zeroes to an ArrayList.
 * Add `error.Overflow` as a possible error to some codepaths, allowing
   usage of `math.intCast`.

closes #9710