Add crypto_generichash_statebytes()

sizeof() is not always an option when accessing the library from
other languages.

src/libsodium/crypto_generichash/crypto_generichash.c

@@ -37,11 +37,18 @@ crypto_generichash_keybytes(void)
     return crypto_generichash_KEYBYTES;
 }
 
-const char *crypto_generichash_primitive(void)
+const char *
+crypto_generichash_primitive(void)
 {
     return crypto_generichash_PRIMITIVE;
 }
 
+size_t
+crypto_generichash_statebytes(void)
+{
+    return sizeof(crypto_generichash_state);
+}
+
 int
 crypto_generichash(unsigned char *out, size_t outlen, const unsigned char *in,
                    unsigned long long inlen, const unsigned char *key,

src/libsodium/include/sodium/crypto_generichash.h

@@ -42,6 +42,8 @@ SODIUM_EXPORT
 const char *crypto_generichash_primitive(void);
 
 typedef crypto_generichash_blake2b_state crypto_generichash_state;
+SODIUM_EXPORT
+size_t  crypto_generichash_statebytes(void);
 
 SODIUM_EXPORT
 int crypto_generichash(unsigned char *out, size_t outlen,

test/default/generichash2.c

@@ -10,6 +10,7 @@ int main(void)
         k[crypto_generichash_KEYBYTES_MAX];
     size_t h, i, j;
 
+    assert(crypto_generichash_statebytes() == sizeof st);
     for (h = 0; h < crypto_generichash_KEYBYTES_MAX; ++h)
         k[h] = h;