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/lib.rs

@@ -7,8 +7,7 @@ pub use merkle_hasher::{Error, MerkleHasher};
 pub use merkleize_padded::merkleize_padded;
 pub use merkleize_standard::merkleize_standard;
 
-use eth2_hashing::{Context, SHA256};
-use eth2_hashing::{ZERO_HASHES, ZERO_HASHES_MAX_INDEX};
+use eth2_hashing::{hash_fixed, ZERO_HASHES, ZERO_HASHES_MAX_INDEX};
 
 pub const BYTES_PER_CHUNK: usize = 32;
 pub const HASHSIZE: usize = 32;
@@ -39,11 +38,7 @@ pub fn merkle_root(bytes: &[u8], minimum_leaf_count: usize) -> Hash256 {
         let mut leaves = [0; HASHSIZE * 2];
         leaves[0..bytes.len()].copy_from_slice(bytes);
 
-        let mut context = Context::new(&SHA256);
-        context.update(&leaves);
-        let digest = context.finish();
-
-        Hash256::from_slice(digest.as_ref())
+        Hash256::from_slice(&hash_fixed(&leaves))
     } else {
         // If there are 3 or more leaves, use `MerkleHasher`.
         let mut hasher = MerkleHasher::with_leaves(leaves);