Use faster byteswapping and avoid alignment-increasing casts.

src/hash.h

@@ -12,7 +12,7 @@
 
 typedef struct {
     uint32_t s[32];
-    unsigned char buf[64];
+    uint32_t buf[16]; /* In big endian */
     size_t bytes;
 } secp256k1_sha256_t;