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).

lighthouse/src/main.rs

@@ -7,6 +7,7 @@ use clap::{App, Arg, ArgMatches};
 use clap_utils::flags::DISABLE_MALLOC_TUNING_FLAG;
 use env_logger::{Builder, Env};
 use environment::EnvironmentBuilder;
+use eth2_hashing::have_sha_extensions;
 use eth2_network_config::{Eth2NetworkConfig, DEFAULT_HARDCODED_NETWORK};
 use lighthouse_version::VERSION;
 use malloc_utils::configure_memory_allocator;
@@ -43,10 +44,13 @@ fn main() {
         .long_version(
             format!(
                 "{}\n\
-                 BLS Library: {}\n\
-                 Specs: mainnet (true), minimal ({}), v0.12.3 ({})",
-                 VERSION.replace("Lighthouse/", ""), bls_library_name(),
-                 cfg!(feature = "spec-minimal"), cfg!(feature = "spec-v12"),
+                 BLS library: {}\n\
+                 SHA256 hardware acceleration: {}\n\
+                 Specs: mainnet (true), minimal ({})",
+                 VERSION.replace("Lighthouse/", ""),
+                 bls_library_name(),
+                 have_sha_extensions(),
+                 cfg!(feature = "spec-minimal"),
             ).as_str()
         )
         .arg(
@@ -205,7 +209,7 @@ fn main() {
             EthSpecId::Mainnet => run(EnvironmentBuilder::mainnet(), &matches, testnet_config),
             #[cfg(feature = "spec-minimal")]
             EthSpecId::Minimal => run(EnvironmentBuilder::minimal(), &matches, testnet_config),
-            #[cfg(any(not(feature = "spec-minimal")))]
+            #[cfg(not(feature = "spec-minimal"))]
             other => {
                 eprintln!(
                     "Eth spec `{}` is not supported by this build of Lighthouse",