Reduce block code

[dhardy]

Corresponds to rust-random/rand_core#34

Needs to be merged with #80 derivative.

[dhardy]

Test failures concern serde (needs further rand_core changes to fix).

[dhardy]

Self-review notes:

Isaac*Rng serde code is inelegant and contains a "TODO" comment, but tests pass and I don't really care about rand_isaac (I'm not sure if anyone does; the main reason I bothered to write this code at all is as a serde test case for a block RNG).

Isaac64Rng has value-breaking changes when invoking next_u32() twice in a row. This is noted in the CHANGELOG and deemed of low importance due to low usage of rand_isaac.

rand_isaac now requires serde_arrays for serde support. This is a small crate depending only on serde.

IsaacArray (internal type) is no longer required.

All other changes are reassuringly boring and minimal with no test changes required.