rt: pull upstream ISAAC code for consistency between 32/64 bit platforms

The "unsigned 4 byte" `ub4`s are actually 8 bytes on 64-bit platforms
which mean that some bits > 2**32 were retained in calculations, these
would then "reappear" after a shift and so the stream of random numbers
would differ on 32 bit vs 64 bit platforms.
rt: use the [u]int[nn]_t types in the RNG.

This means that `ub4`s are always 4 bytes, rather than being 8 bytes on
x64. (Suggested but not implemented by upstream: "Porting it to a 64-bit
machine [...] may just need an adjustment of the definition of ub4")
auto merge of #6058 : huonw/rust/rt-isaac-update, r=graydon

The "unsigned 4 byte" `ub4`s are actually 8 bytes on 64-bit platforms
which mean that some bits > 2**32 were retained in calculations, these
would then "reappear" after a right shift and so the stream of random numbers
would differ on 32 bit vs 64 bit platforms.

http://burtleburtle.net/bob/c/randport.c