Don't use reserved identifiers memczero and benchmark_verify_t

As identified in #829 and #833. Fixes #829.

Since we touch this anyway, this commit additionally makes the
identifiers in the benchmark files a little bit more consistent.

src/util.h

@@ -202,7 +202,7 @@ static SECP256K1_INLINE void *manual_alloc(void** prealloc_ptr, size_t alloc_siz
 #endif
 
 /* Zero memory if flag == 1. Flag must be 0 or 1. Constant time. */
-static SECP256K1_INLINE void memczero(void *s, size_t len, int flag) {
+static SECP256K1_INLINE void secp256k1_memczero(void *s, size_t len, int flag) {
     unsigned char *p = (unsigned char *)s;
     /* Access flag with a volatile-qualified lvalue.
        This prevents clang from figuring out (after inlining) that flag can