Pack u128 in the compiler to mitigate new alignment This is based on #116672, adding a new `#[repr(packed(8))]` wrapper on `u128` to avoid changing any of the compiler's size assertions. This is needed in two places: * `SwitchTargets`, otherwise its `SmallVec<[u128; 1]>` gets padded up to 32 bytes. * `LitKind::Int`, so that entire `enum` can stay 24 bytes. * This change definitely has far-reaching effects though, since it's public.