crypto/sha256: add benchmark.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/crypto/sha256/benchmarks/Makefile

+CCANDIR := ../../../../
+CFLAGS := -Wall -I$(CCANDIR) -O3 -flto
+LDFLAGS := -O3 -flto
+
+double-sha-bench: double-sha-bench.o ccan-time.o ccan-crypto-sha256.o
+
+ccan-crypto-sha256.o: $(CCANDIR)/ccan/crypto/sha256/sha256.c
+	$(CC) $(CFLAGS) -c -o $@ $<
+ccan-time.o: $(CCANDIR)/ccan/time/time.c
+	$(CC) $(CFLAGS) -c -o $@ $<

ccan/crypto/sha256/benchmarks/double-sha-bench.c

+/* Bitcoin does a lot of SHA of SHA.  Benchmark that. */
+#include <ccan/crypto/sha256/sha256.h>
+#include <ccan/time/time.h>
+#include <stdio.h>
+
+int main(int argc, char *argv[])
+{
+	struct timeabs start;
+	struct timerel diff;
+	size_t i, n;
+	struct sha256 h;
+
+	n = atoi(argv[1] ? argv[1] : "1000000");
+	sha256(&h, &n, sizeof(n));
+	start = time_now();
+	for (i = 0; i < n; i++)
+		sha256(&h, &h, sizeof(h));
+	diff = time_divide(time_between(time_now(), start), n);
+	printf("Hashing time for %02x%02x%02x%02x%02x%02x... is %llu nsec\n",
+	       h.u.u8[0], h.u.u8[1], h.u.u8[2], h.u.u8[3], h.u.u8[4], h.u.u8[5],
+	       (unsigned long long)time_to_nsec(diff));
+	return 0;
+}