Index the [u64x2] array as a [u64].

This is far more efficient than splitting into two parity cases.

Before:

    test bench_mt19937_1_000_000_rands ... bench:   2246224 ns/iter (+/- 103394)

After:

    test bench_mt19937_1_000_000_rands ... bench:   1920277 ns/iter (+/- 197514)
Avoid inlining the rarely-used large gen_rand_all.

It's a big chunk of code that is rarely executed, so keeping it out of
line for the common case provides a significant speed-up.

Before:

    test bench_mt19937_1_000_000_rands ... bench:   1920277 ns/iter (+/- 197514)

After:

    test bench_mt19937_1_000_000_rands ... bench:   1567069 ns/iter (+/- 277793)
Merge branch 'faster-index' of https://github.com/huonw/dsfmt-rs into huonw-faster-index
Merge branch 'huonw-faster-index'