Align Poplar1 and Idpf with draft-irtf-cfrg-vdaf-08

[cjpatton]

Partially addresses #839.

• Modify Idpf::extend() by stealing control bits from the seeds
• Bind the nonce to the shard randomness in Poplar1::shard()

[cjpatton]

This has lead to a 15% speed-up on my laptop :)
fyi/ @schoppmp 👏

[cjpatton/839-poplar1][~/github.com/divviup/libprio-rs/src]$ cargo bench --all-features -- poplar1
   Compiling prio v0.15.3 (/Users/christopherpatton/github.com/divviup/libprio-rs)
    Finished bench [optimized] target(s) in 5.30s
     Running benches/cycle_counts.rs (/Users/christopherpatton/github.com/divviup/libprio-rs/target/release/deps/cycle_counts-6cc2a04ac9675b6e)
Unexpected error while launching valgrind. Error: No such file or directory (os error 2)
     Running benches/speed_tests.rs (/Users/christopherpatton/github.com/divviup/libprio-rs/target/release/deps/speed_tests-47137adb65076f54)
Gnuplot not found, using plotters backend
poplar1_shard/16        time:   [61.659 µs 61.723 µs 61.798 µs]
                        thrpt:  [31.605 KiB/s 31.643 KiB/s 31.676 KiB/s]
                 change:
                        time:   [-12.578% -12.390% -12.180%] (p = 0.00 < 0.05)
                        thrpt:  [+13.869% +14.142% +14.387%]
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  6 (6.00%) high mild
  3 (3.00%) high severe
poplar1_shard/128       time:   [395.07 µs 395.74 µs 396.52 µs]
                        thrpt:  [39.405 KiB/s 39.483 KiB/s 39.550 KiB/s]
                 change:
                        time:   [-14.631% -14.373% -14.095%] (p = 0.00 < 0.05)
                        thrpt:  [+16.407% +16.785% +17.138%]
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  2 (2.00%) high mild
  2 (2.00%) high severe
poplar1_shard/256       time:   [775.50 µs 776.96 µs 779.08 µs]
                        thrpt:  [40.111 KiB/s 40.221 KiB/s 40.297 KiB/s]
                 change:
                        time:   [-16.012% -15.713% -15.424%] (p = 0.00 < 0.05)
                        thrpt:  [+18.237% +18.642% +19.065%]
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  5 (5.00%) high mild
  4 (4.00%) high severe

poplar1_prepare_init/16 time:   [547.42 µs 549.23 µs 551.11 µs]
                        change: [-10.864% -10.534% -10.220%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  1 (1.00%) low mild
  4 (4.00%) high mild
poplar1_prepare_init/128
                        time:   [3.8224 ms 3.8301 ms 3.8382 ms]
                        change: [-14.531% -14.322% -14.075%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  4 (4.00%) high mild
  1 (1.00%) high severe
poplar1_prepare_init/256
                        time:   [7.4107 ms 7.4299 ms 7.4496 ms]
                        change: [-15.341% -15.039% -14.736%] (p = 0.00 < 0.05)
                        Performance has improved.