Issue #4709 RFC? refactor and reseed rand::Rng

[cpeterso]

For Issue #4709:

c531506 rt: rand.rs expects rust_next() to return uint32_t, not size_t

rand.rs expects rustrt::rand_next() to return u32, but the rand_next() C function returns size_t: https://github.com/mozilla/rust/blob/ca71c6ec5bb8106121cbe33eec5a6a9ca7786039/src/libcore/rand.rs#L34

f4320b6 move isaac RNG utility functions to new rust_rng.cpp file
665e900 encapsulate isaac RNG in rust_rng struct

Move isaac's randctx into a rust_rng struct to make names similar to rand::Rng function names and prepare for auto-reseeding in the next commit.

9a78dc9 reseed rust_rng after generating 32KB

Precedents from other languages:

• Haskell's GenAutoReseed generator reseeds itself after generating 32KB: http://hackage.haskell.org/packages/archive/DRBG/0.1.2/doc/html/Crypto-Random-DRBG.html#t:GenAutoReseed
• Go's RNG reseeds itself after generating 1MB: https://code.google.com/p/go/source/browse/src/pkg/crypto/rand/rand_unix.go?name=go1.0.3#94

9a76d71 don't deplete RNG entropy when there is only one runnable task

rust_sched_loop::schedule_task() unnecessarily calls isaac_rand() for the common case when there is only 1 runnable task, thus depleting RNG entropy and incurring unnecessary overhead.

[cpeterso]

btw, most of this code could be deleted if the task_rng Rng was rewritten in Rust. The only other code that depends on the isaac C code is rust_schec_loop::schedule_task(), which doesn't need a strong CSPRNG or, if the task scheduler is rewritten in Rust, could use the Rust task_rng.

[graydon]

I concur that much of this stuff should move into rust at some point; but this is a helpful short-term fix for misbehavior and I greatly appreciate it!