feat: support hashtree sha256 lib

[estensen]

Stacked on #169

Just enabling feature flags give much better perf. Step towards #156 but more optimizations possible as mentioned in #161 (comment). But I don't think lack of vectorized/parallel pipelining should prevent this from being merged. This is a major step towards getting this library performant enough so it can be used.

default sha256

sudo cargo bench --bench compute_proof && sudo cargo bench --bench compute_proof
...
Prove Benchmark - File: benches/21315748.json - size 247/123
                        time:   [153.78 ms 155.64 ms 156.80 ms]
                        change: [-1.7347% -0.5531% +0.6920%] (p = 0.42 > 0.05)
                        No change in performance detected.

Prove Benchmark - File: benches/21327802.json - size 261/130
                        time:   [155.16 ms 156.10 ms 156.81 ms]
                        change: [-1.5089% -0.5273% +0.8751%] (p = 0.46 > 0.05)
                        No change in performance detected.

sha256-asm feature flag enabled

sudo cargo bench --bench compute_proof && sudo cargo bench --bench compute_proof --features sha256-asm
...
Prove Benchmark - File: benches/21315748.json - size 247/123
                        time:   [56.672 ms 57.170 ms 57.664 ms]
                        change: [-63.290% -62.915% -62.534%] (p = 0.00 < 0.05)
                        Performance has improved.

Prove Benchmark - File: benches/21327802.json - size 261/130
                        time:   [57.677 ms 58.089 ms 58.523 ms]
                        change: [-63.573% -63.007% -62.541%] (p = 0.00 < 0.05)
                        Performance has improved.

hashtree

sudo cargo bench --bench compute_proof && sudo cargo bench --bench compute_proof --features hashtree
...
Prove Benchmark - File: benches/21315748.json - size 247/123
                        time:   [48.933 ms 50.096 ms 51.119 ms]
                        change: [-15.087% -13.401% -11.414%] (p = 0.00 < 0.05)
                        Performance has improved.

Prove Benchmark - File: benches/21327802.json - size 261/130
                        time:   [48.718 ms 49.614 ms 51.459 ms]
                        change: [-13.992% -11.575% -8.9135%] (p = 0.00 < 0.05)
                        Performance has improved.