Fix SmallRng::from_seed causing a compile error for 32-bit targets

[neonmoe]

While trying to build the crate with armv6k-nintendo-3ds as the target, I got the error:

error[E0308]: mismatched types
   --> /home/jens/.cargo/git/checkouts/piccolo-12b9ffbffc01d1c4/ce709eb/src/stdlib/math.rs:348:65
    |
348 |                         *rng.borrow_mut() = SmallRng::from_seed(seed);
    |                                             ------------------- ^^^^ expected an array with a size of 16, found one with a size of 32
    |                                             |
    |                                             arguments to this function are incorrect
    |
note: associated function defined here
   --> /home/jens/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rand_core-0.6.4/src/lib.rs:319:8
    |
319 |     fn from_seed(seed: Self::Seed) -> Self;
    |        ^^^^^^^^^

For more information about this error, try `rustc --explain E0308`.

While a bit puzzling, the SmallRng docs spelled it out clearly enough ("The current algorithm is Xoshiro256PlusPlus on 64-bit platforms and Xoshiro128PlusPlus on 32-bit platforms") for me to find a pretty concise fix, so here it is.