consensus/ethash: implement faster difficulty calculators (#21976)

This PR adds re-written difficulty calculators, which are based on uint256. It also adds a fuzzer + oss-fuzz integration for the new fuzzer. It does differential fuzzing between the new and old calculators.

Note: this PR does not actually enable the new calculators.

tests/fuzzers/difficulty/debug/main.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+
+	"github.com/ethereum/go-ethereum/tests/fuzzers/difficulty"
+)
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Fprintf(os.Stderr, "Usage: debug <file>")
+		os.Exit(1)
+	}
+	crasher := os.Args[1]
+	data, err := ioutil.ReadFile(crasher)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error loading crasher %v: %v", crasher, err)
+		os.Exit(1)
+	}
+	difficulty.Fuzz(data)
+}