Boot sha2-asm

Stick with pure Rust until someone can write a benchmark that
demonstrates that sha2-asm adds value. If we go with a GPU
implementation first, we may never need to do that.

README.md

@@ -128,5 +128,5 @@ $ rustup install nightly
 Run the benchmarks:
 
 ```bash
-$ cargo +nightly bench --features="asm,unstable"
+$ cargo +nightly bench --features="unstable"
 ```