Fix integer overflow -> undefined behavior in LockFreeFrozenVec capacity

[kevinmehall]

The following example currently overflows usize in the calculation of the allocation size, writes into an allocation that is actually zero bytes, and segfaults:

use elsa::sync::*;

fn main() {
    let v = LockFreeFrozenVec::<u64>::with_capacity(1<<(64 - 3));
    loop { v.push(1); }
}

This changes it to use Layout::array, which checks for overflow.

[Manishearth]

nice catch!

(I really should mark the lock free stuff as experimental)