Issue #4709 RFC: reseed task_rng

[cpeterso]

@graydon for issue #4709.

The first change (commit 294b4a9) is straightforward. It adds a new isaac_reseed() function and uninlines isaac_seed() and isaac_init().

The second change (commit 2ef8a32) is more questionable. It adds a rust_sched_loop::idle() function, which is called when rust_sched_driver::start_main_loop() has no runnable tasks. idle() checks whether rctx has drained its entropy pool, i.e. generated more than 256 random uint32_ts. If not reseeded, rctx will be stirring the same entropy bits to generate new random numbers.

Possible improvements:

• Add a minimum and/or maximum reseed frequency to idle(). For example, Schneier and Ferguson's Fortuna PRNG doesn't reseed more often 10 times per second.
• Reseed less frequently than 256 random numbers (isaac's RANDSIZ)?
• If Rust's task scheduler supported alarm tasks to be run after a specified timeout, they could be used for periodic cleanup work like reseeding the RNG.

This change also adds a fast path to rust_sched_loop::schedule_task() when there is only 1 runnable task to avoid depleting task_rng's precious entropy.

[brson]

The second commit reseeds the scheduler's random number generator, which is probably desirable, but is not the same as the TLS generator mentioned in #4709. That rng is created by core::rand::task_rng.

[cpeterso]

doh!