hash: Make code agnostic of endianness

Recent compilers compile the two new functions to very efficient code
on various platforms. In particular, already GCC >= 5 and clang >= 5
understand do this for the read function, which is the one critical
for performance (called 16 times per SHA256 transform).

Fixes #1080.

src/hash.h

@@ -12,7 +12,7 @@
 
 typedef struct {
     uint32_t s[8];
-    uint32_t buf[16]; /* In big endian */
+    unsigned char buf[64];
     uint64_t bytes;
 } secp256k1_sha256;