std::rand: simplify/safe-ify the default Rng.fill_bytes.

The `&[u8]` -> `&[u64]` and `&[u32]` casts were not nice: they ignored
alignment requirements and are generally very unsafe.
std::rand: use "nothing up your sleeve numbers" for ISAAC tests.

There's no value in using the "random" numbers, when nothing up your
sleeve numbers are perfectly serviceable.

http://en.wikipedia.org/wiki/Nothing_up_my_sleeve_number
std::rand: seed ISAAC with no transmutes.

Slice transmutes are now (and, really, always were) dangerous, so we
avoid them and do the (only?) non-(undefined behaviour in C) pointer
cast: casting to *u8.
auto merge of #10015 : huonw/rust/minor-fixes, r=alexcrichton

- Use ["nothing up my sleeve numbers"](http://en.wikipedia.org/wiki/Nothing_up_my_sleeve_number) for the ISAAC tests.
- Replace the default implementation of `Rng.fill_bytes` with something that doesn't try to do bad things with `transmute` and vectors just for the sake of a little speed.
- Replace the transmutes used to seed the ISAAC RNGs with calls into `vec::raw`.