POC: AVX2 sha256

Before:
```
test bench1_10    ... bench:          39 ns/iter (+/- 2) = 256 MB/s
test bench2_100   ... bench:         349 ns/iter (+/- 23) = 286 MB/s
test bench3_1000  ... bench:       3,412 ns/iter (+/- 11) = 293 MB/s
test bench4_10000 ... bench:      34,084 ns/iter (+/- 3,183) = 293 MB/s
```

After:
```
itest bench1_10    ... bench:          27 ns/iter (+/- 1) = 370 MB/s
test bench2_100   ... bench:         232 ns/iter (+/- 5) = 431 MB/s
test bench3_1000  ... bench:       2,082 ns/iter (+/- 85) = 480 MB/s
test bench4_10000 ... bench:      20,543 ns/iter (+/- 2,686) = 486 MB/s
```

sha2/build.rs

@@ -5,7 +5,7 @@ fn main() {
     let (sha256_path, sha512_path) = if target_arch == "x86" {
         ("src/sha256_x86.S", "src/sha512_x86.S")
     } else if target_arch == "x86_64" {
-        ("src/sha256_x64.S", "src/sha512_x64.S")
+        ("src/sha256-avx2-asm.S", "src/sha512_x64.S")
     } else if target_arch == "aarch64" {
         build256.flag("-march=armv8-a+crypto");
         ("src/sha256_aarch64.S", "")

sha2/src/lib.rs

@@ -14,15 +14,17 @@
 compile_error!("crate can only be used on x86, x86-64 and aarch64 architectures");
 
 #[link(name = "sha256", kind = "static")]
+#[allow(dead_code)]
 extern "C" {
     fn sha256_compress(state: &mut [u32; 8], block: &[u8; 64]);
+    fn sha256_transform_rorx(state: &mut [u32; 8], block: *const [u8; 64], num_blocks: usize);
 }
 
 /// Safe wrapper around assembly implementation of SHA256 compression function
 #[inline]
 pub fn compress256(state: &mut [u32; 8], blocks: &[[u8; 64]]) {
-    for block in blocks {
-        unsafe { sha256_compress(state, block) }
+    if !blocks.is_empty() {
+        unsafe { sha256_transform_rorx(state, blocks.as_ptr(), blocks.len()) }
     }
 }