Split memclear into two versions

secp256k1_memclear has the side effect of undefining bytes for
valgrind checks. In some cases, we may want to zero bytes
but allow subsequent reads. So we split memclear into
memclear_explicit, which makes no guarantees about the content
of the buffer on return, and memzero_explicit, which guarantees
zero value on return.

Change the memset in partial_sign to use memzero_explicit.

src/scalar_impl.h

@@ -28,7 +28,7 @@ static const secp256k1_scalar secp256k1_scalar_one = SECP256K1_SCALAR_CONST(0, 0
 static const secp256k1_scalar secp256k1_scalar_zero = SECP256K1_SCALAR_CONST(0, 0, 0, 0, 0, 0, 0, 0);
 
 SECP256K1_INLINE static void secp256k1_scalar_clear(secp256k1_scalar *r) {
-    secp256k1_memclear(r, sizeof(secp256k1_scalar));
+    secp256k1_memclear_explicit(r, sizeof(secp256k1_scalar));
 }
 
 static int secp256k1_scalar_set_b32_seckey(secp256k1_scalar *r, const unsigned char *bin) {