Use hardware acceleration for SHA256 (#2426)

## Proposed Changes

Modify the SHA256 implementation in `eth2_hashing` so that it switches between `ring` and `sha2` to take advantage of [x86_64 SHA extensions](https://en.wikipedia.org/wiki/Intel_SHA_extensions). The extensions are available on modern Intel and AMD CPUs, and seem to provide a considerable speed-up: on my Ryzen 5950X it dropped state tree hashing times by about 30% from 35ms to 25ms (on Prater).

## Additional Info

The extensions became available in the `sha2` crate [last year](https://www.reddit.com/r/rust/comments/hf2vcx/ann_rustcryptos_sha1_and_sha2_now_support/), and are not available in Ring, which uses a [pure Rust implementation of sha2](https://github.com/briansmith/ring/blob/main/src/digest/sha2.rs). Ring is faster on CPUs that lack the extensions so I've implemented a runtime switch to use `sha2` only when the extensions are available. The runtime switching seems to impose a miniscule penalty (see the benchmarks linked below).

consensus/tree_hash/src/merkleize_padded.rs

@@ -1,5 +1,5 @@
 use super::{get_zero_hash, Hash256, BYTES_PER_CHUNK};
-use eth2_hashing::{hash, hash32_concat};
+use eth2_hashing::{hash32_concat, hash_fixed};
 
 /// Merkleize `bytes` and return the root, optionally padding the tree out to `min_leaves` number of
 /// leaves.
@@ -79,7 +79,7 @@ pub fn merkleize_padded(bytes: &[u8], min_leaves: usize) -> Hash256 {
         // Hash two chunks, creating a parent chunk.
         let hash = match bytes.get(start..start + BYTES_PER_CHUNK * 2) {
             // All bytes are available, hash as usual.
-            Some(slice) => hash(slice),
+            Some(slice) => hash_fixed(slice),
             // Unable to get all the bytes, get a small slice and pad it out.
             None => {
                 let mut preimage = bytes
@@ -87,7 +87,7 @@ pub fn merkleize_padded(bytes: &[u8], min_leaves: usize) -> Hash256 {
                     .expect("`i` can only be larger than zero if there are bytes to read")
                     .to_vec();
                 preimage.resize(BYTES_PER_CHUNK * 2, 0);
-                hash(&preimage)
+                hash_fixed(&preimage)
             }
         };